Answer key · Unit 10

Control and coordination.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 7

Lesson 1 of 7 · About 25 minutes

Nerve signals

How a nerve cell passes a message on: the resting potential, the threshold, an impulse that is all or nothing, strength coded as rate, and myelin that speeds the impulse a hundredfold.

Open the lesson

The 7 steps and their answers

Step 1 of 7
Open this step
A cell at rest
Practice · Calculate
Question
Stimulus is fixed at 0%. What is the voltage across the membrane, inside against outside, when the nerve cell is at rest? Answer in millivolts.

Correct answer
-70 mV ± 2 mV
Setup, choices and feedback
The Scene at this step
Stimulus
0 %fixed
Myelin sheath
on
Show the ions
Off
Accepted within
± 2 mV
Computed from the Scene at this step’s setting
The highest membrane voltage at the start of the axon: +40 mV for every pulse, −70 mV at rest, under −55 mV below the threshold/nodes/cell/outputs/peakVoltage
Any unit of the same quantity is accepted; it is converted before checking.
Feedback for a right answer
−70 mV: the inside is slightly negative. This is the resting potential, and an impulse is a brief flip of it.

Hint
At rest the inside of the cell is negative compared with the outside.

Worked solution
1. At rest, pumps in the membrane push sodium out and bring potassium in, and potassium leaks out.
2. The result: the inside is negative compared with the outside.
3. That voltage is about −70 mV, the resting potential.
Mind the sign: +70 mV would mean a positive inside, which is not the resting state.
Step 2 of 7
Open this step
The threshold
Practice · Reach the target
Question
Myelin sheath is fixed. Raise Stimulus step by step, and find the weakest stimulus that makes the cell fire. Leave it there and press Check.

Goal
- 1 ± 0.1
  At the start of the step: 0
- 0.625 Hz ± 0.02 Hz
  At the start of the step: 0 Hz
Setup, choices and feedback
The Scene at this step
Stimulus
0 %
Myelin sheath
onfixed
Show the ions
Off
Read from the Scene at
1 when the stimulus reaches the threshold and the neuron fires; else 0/nodes/cell/outputs/fires
Read from the Scene at
Pulses a second, as the Scene fires them (slowed with it); 0 below the threshold/nodes/cell/outputs/pulseRate
One setting that reaches it
Stimulus
40 %
Feedback for a right answer
40%: this cell’s threshold. At 35% the membrane voltage rises a little but does not reach −55 mV, and no impulse sets off.

Feedback for a wrong answer
Not yet. If the cell fires, try a weaker stimulus; if not, a stronger one.

Hint
Below the threshold only the dendrites flicker. At the threshold the cell body lights and an impulse sets off.

Worked solution
1. Start at 0% and raise it in steps of 5%.
2. Up to 35% no impulse sets off: the voltage rises, but does not reach the threshold.
3. At 40% the cell fires its first impulses, slowly.
Every stimulus above 40% fires too, but faster. The weakest one that fires is the threshold.
Step 3 of 7
Open this step
All or nothing
Practice · Predict, then look
Question
At a stimulus of 50% each impulse peaks at +40 mV. What will the peak be when the stimulus rises to 100%?

Correct answer
- Correct The same peak, +40 mV
Setup, choices and feedback
The Scene at this step
Stimulus
50 %fixed
Myelin sheath
onfixed
Show the ions
Off
Choices
- Correct The same peak, +40 mV
- Wrong About twice, +80 mV
  Misconception: Thinks a stronger stimulus makes a bigger nerve impulse.
  Feedback from the catalogue
  A nerve impulse is all or nothing: below the threshold there is none, and above it it is always the same size, up to +40 mV. A stronger stimulus does not make the impulse bigger; it makes the cell fire more impulses a second. Strength is coded in the rate, not the height.
- Wrong A little higher
  Misconception: Thinks a stronger stimulus makes a bigger nerve impulse.
  Feedback from the catalogue
  A nerve impulse is all or nothing: below the threshold there is none, and above it it is always the same size, up to +40 mV. A stronger stimulus does not make the impulse bigger; it makes the cell fire more impulses a second. Strength is coded in the rate, not the height.
Once the prediction is locked in, the Scene changes
Stimulus
100 %fixed
The Scene then shows: 40 mV · The highest membrane voltage at the start of the axon: +40 mV for every pulse, −70 mV at rest, under −55 mV below the threshold/nodes/cell/outputs/peakVoltage
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The same +40 mV. Once the gates open, the sodium rushing in sets the peak, not the stimulus. What changed is the number of impulses: count how many set off now.

Worked solution
1. Above the threshold the sodium gates open, and sodium flows in until the voltage reaches about +40 mV.
2. That happens the same way in every impulse, whatever the stimulus.
3. So the peak at 100% equals the peak at 50%.
If you predicted a higher peak, you put the strength in the height of the impulse. It is in the rate.
Step 4 of 7
Open this step
Strength as rate
Practice · Predict, then look
Question
A stimulus of 45%, a little over the threshold. What will change when the stimulus rises to 90%?

Correct answer
- Correct The cell will fire more impulses a second
Setup, choices and feedback
The Scene at this step
Stimulus
45 %fixed
Myelin sheath
onfixed
Show the ions
Off
Choices
- Correct The cell will fire more impulses a second
- Wrong Each impulse will be bigger
  Misconception: Thinks a stronger stimulus makes a bigger nerve impulse.
  Feedback from the catalogue
  A nerve impulse is all or nothing: below the threshold there is none, and above it it is always the same size, up to +40 mV. A stronger stimulus does not make the impulse bigger; it makes the cell fire more impulses a second. Strength is coded in the rate, not the height.
- Wrong The impulses will travel faster along the axon
  Feedback
  The speed along the axon depends on the myelin and the axon’s width, not on the stimulus.
Once the prediction is locked in, the Scene changes
Stimulus
90 %fixed
The Scene then shows: 1.791 Hz · Pulses a second, as the Scene fires them (slowed with it); 0 below the threshold/nodes/cell/outputs/pulseRate
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
In the Scene the rate rises from about 0.7 to about 1.8 impulses a second, more than double (the Scene is slowed; a real nerve cell fires tens to hundreds of impulses a second). That is how the brain tells a light touch from a hard press.

Worked solution
1. Each impulse is all or nothing, so its size cannot carry the strength.
2. A strong stimulus brings the cell back to threshold sooner after each impulse.
3. So it fires impulses more often.
The speed of each impulse along the axon does not change.
Step 5 of 7
Open this step
What travels along the axon
Practice · Choose
Question
Show the ions is On. What moves along the axon as an impulse passes?

Correct answer
- Correct A wave of gates opening in the membrane: sodium ions flow in, then potassium ions flow out
Setup, choices and feedback
The Scene at this step
Stimulus
90 %
Myelin sheath
on
Show the ions
On
Choices
Shown in a random order.
- Correct A wave of gates opening in the membrane: sodium ions flow in, then potassium ions flow out
- Wrong Electrons flowing from one end of the axon to the other, as in an electric wire
  Misconception: Thinks a nerve impulse is electrons flowing along the axon like current in a copper wire.
  Feedback from the catalogue
  An axon is not a copper wire. The impulse is a wave travelling along the cell membrane: gates in the membrane open, sodium ions flow in and flip the voltage of that patch, and the flip opens the gates in the next patch. Behind the wave potassium ions flow out and restore the voltage. That is why an impulse is far slower than electricity in a wire: one to a hundred metres a second.
- Wrong A chemical that flows inside the axon to its end
  Feedback
  Chemical messengers act only at the end, at the synapse. Along the axon itself the impulse is an electrical wave in the membrane.
Feedback for a right answer
Right. Each ion moves only across the membrane, a few micrometres; what travels along the axon is the flip of the voltage, like a wave through a stadium crowd.

Hint
Watch the ions crossing the membrane just ahead of the impulse and behind it.

Worked solution
1. At the threshold sodium gates open, and sodium flows in and turns the inside positive.
2. The flip opens the gates in the next patch, and so on along the axon.
3. Behind the wave potassium flows out and brings the voltage back to rest.
If you chose electrons in a wire, you were thinking of electricity in metal. In a nerve cell the charges are ions, and they move across the membrane, not along the axon.
Step 6 of 7
Open this step
How fast
Practice · Calculate
Question
A nerve 1 m long carries an impulse from the toe to the spinal cord. With myelin the impulse travels at about 100 m/s. How many milliseconds does the trip take?

Correct answer
10 ms ± 3%
Setup, choices and feedback
The Scene at this step
Stimulus
90 %
Myelin sheath
onfixed
Show the ions
Off
Accepted within
± 3%
Computed from the Scene at this step’s setting
Time for a pulse to cover nerveLength/nodes/cell/outputs/travelTime
Any unit of the same quantity is accepted; it is converted before checking.
Feedback for a right answer
$\dfrac{1}{100} = 0.01$ ⁠ second, that is 10 ms. That is why a stubbed toe is felt almost at once.

Hint
Time = distance ÷ speed. A second is 1000 ms.

Worked solution
1. The givens: distance 1 m, speed 100 m/s.
2. Time = distance ÷ speed = $1 \div 100 = 0.01$ ⁠ second.
3. In milliseconds: $0.01 \times 1000 = 10$ ⁠.
The answer: 10 ms. If you got $100$ ⁠, you multiplied the speed by the distance instead of dividing.
Step 7 of 7
Open this step
Without myelin
Practice · Predict, then look
Question
With myelin the impulse covers the metre in 10 ms. What will happen when you switch off Myelin sheath?

Correct answer
- Correct The impulse will arrive about a hundred times later, after about a second
Setup, choices and feedback
The Scene at this step
Stimulus
90 %
Myelin sheath
onfixed
Show the ions
Off
Choices
- Correct The impulse will arrive about a hundred times later, after about a second
- Wrong The impulse will arrive at the same time, but weaker
  Misconception: Thinks myelin makes the nerve impulse stronger rather than faster.
  Feedback from the catalogue
  Myelin does not make the impulse bigger: the impulse is always the same size. The fatty sheath insulates the axon, so the impulse jumps from gap to gap instead of creeping patch by patch, and it travels much faster, about a hundred metres a second instead of about one.
- Wrong The impulse will arrive a little later, about three times
  Feedback
  In the Scene the bare impulse looks only three times slower, so both can be watched. In real nerves a bare axon is tens to hundreds of times slower.
Once the prediction is locked in, the Scene changes
Myelin sheath
offfixed
The Scene then shows: 1 s · Time for a pulse to cover nerveLength/nodes/cell/outputs/travelTime
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About a whole second: a bare axon conducts at about a metre a second. That is why diseases that damage myelin, such as multiple sclerosis, slow and scramble signals.

Worked solution
1. With myelin: $1 \div 100 = 0.01$ ⁠ second.
2. Without myelin the impulse creeps patch by patch, about 1 m/s.
3. The same metre: $1 \div 1 = 1$ ⁠ second, a hundred times longer.
If you predicted a weaker impulse, you thought myelin boosts it. The impulse is always the same size; the myelin sets the speed.

Lesson 2 of 7 · About 25 minutes

Homeostasis and feedback

How the body holds 37 degrees: a cool room and a freezing night, what happens with the thermostat switched off, why sweat cools, the band where you neither shiver nor sweat, and what negative feedback is.

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The 6 steps and their answers

Step 1 of 6
Open this step
Thirty-seven degrees
Practice · Calculate
Question
A person stands for an hour in a room where Air temperature is 283 K, that is 10 °C. What is their core temperature at the end of the hour, in degrees Celsius?

Correct answer
36.91 °C ± 0.3 °C
Setup, choices and feedback
The Scene at this step
Air temperature
283.1 Kfixed
Exercise
100 Wfixed
Thermostat
Workingfixed
Labels
offdefault
Zoom to the skin
Person
Accepted within
± 0.3 °C
Computed from the Scene at this step’s setting
Core temperature after the hour, K/nodes/person/outputs/core
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 10 °C ± 3 °C
  Misconception: Thinks the body's core temperature follows the air around it, rising in heat and falling in cold.
  Feedback from the catalogue
  The skin does cool in a cold room, but the core temperature, of the brain and the inner organs, stays near 37 °C in almost any room. The body corrects every drift: in the cold the skin’s vessels narrow and the muscles shiver, and in the heat the vessels widen and the skin sweats.
Feedback for a right answer
About 36.9 °C, almost as in a pleasant room. The skin cools to about 28 °C, its vessels narrow and the muscles shiver, but the core is held.

Hint
Read the thermometer in the Scene, or the core curve on the chart.

Worked solution
1. The hypothalamus senses that the skin is cooling and the blood reaching it is a little cooler.
2. It narrows the skin’s vessels, so less heat reaches the surface, and starts shivering, which makes heat.
3. Heat in roughly equals heat out, and the core stays near 36.9 °C.
If you answered 10 °C, you assumed the body cools to room temperature.
Step 2 of 6
Open this step
A freezing night
Practice · Predict, then look
Question
Now Air temperature will drop to 263 K, a freezing night of −10 °C, for an hour. What will happen to the core temperature?

Correct answer
- Correct It will stay close to 37 °C: the body shivers and narrows the skin’s vessels
Setup, choices and feedback
The Scene at this step
Air temperature
298.1 Kfixed
Exercise
100 Wfixed
Thermostat
Workingfixed
Labels
offdefault
Zoom to the skin
Person
Choices
- Correct It will stay close to 37 °C: the body shivers and narrows the skin’s vessels
- Wrong It will fall a long way, toward the cold outside
  Misconception: Thinks the body's core temperature follows the air around it, rising in heat and falling in cold.
  Feedback from the catalogue
  The skin does cool in a cold room, but the core temperature, of the brain and the inner organs, stays near 37 °C in almost any room. The body corrects every drift: in the cold the skin’s vessels narrow and the muscles shiver, and in the heat the vessels widen and the skin sweats.
- Wrong It will rise, because shivering warms
  Feedback
  Shivering makes heat to make up for what is lost, not to warm beyond it. Negative feedback aims at the set point, not past it.
Once the prediction is locked in, the Scene changes
Air temperature
263.1 Kfixed
The Scene then shows: 36.78 °C · Core temperature after the hour, K/nodes/person/outputs/core
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About 36.8 °C. Shivering runs flat out, about 200 W, and the skin’s vessels are almost shut. In cold like this it is barely enough: the body is losing a little heat, and over more hours the core would start to fall.

Worked solution
1. The air is very cold, and the body loses heat fast through the skin.
2. The hypothalamus answers: the skin’s vessels narrow and the muscles shiver.
3. The heat from shivering replaces nearly all the heat lost, and the core falls by only a tenth of a degree.
If you predicted the core would fall toward the cold, you thought the body follows the air. It corrects against it.
Step 3 of 6
Open this step
The thermostat
Practice · Predict, then look
Question
The same freezing night. Now Thermostat will switch to Switched off: skin blood flow, sweating and shivering stay as they were in a pleasant room. What will happen to the core within an hour?

Correct answer
- Correct It will fall by several degrees
Setup, choices and feedback
The Scene at this step
Air temperature
263.1 Kfixed
Exercise
100 Wfixed
Thermostat
Workingfixed
Labels
offdefault
Zoom to the skin
Person
Choices
- Correct It will fall by several degrees
- Wrong It will stay at 37 °C: that is the body’s natural temperature
  Misconception: Thinks body temperature stays at 37 °C by itself, with nothing working to keep it there.
  Feedback from the catalogue
  37 °C is not a resting state: the body is losing heat to the air all the time, and making heat all the time. Its temperature stays steady only because the hypothalamus measures it and changes skin blood flow, sweating and shivering until the heat coming in equals the heat going out. Without that thermostat the core would drift within an hour.
Once the prediction is locked in, the Scene changes
Thermostat
Switched offfixed
The Scene then shows: 33.74 °C · Core temperature after the hour, K/nodes/person/outputs/core
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
To about 33.7 °C, more than three degrees in one hour: that is already hypothermia. With nothing measuring and correcting, the body loses more heat than it makes, and the core just drifts.

Worked solution
1. Without the thermostat, the skin keeps getting warm blood as in a pleasant room, and there is no shivering.
2. The body makes about 100 W and loses more than 300 W in this cold.
3. The difference comes out of the heat stored in the body, and the core falls to about 33.7 °C within an hour.
If you predicted the core would stay at 37 °C, you thought it was steady by itself. It is steady only because something keeps correcting it.
Step 4 of 6
Open this step
Hotter than the skin
Practice · Predict, then look
Question
Air temperature will rise to 313 K, that is 40 °C: hotter than the skin. How will the body lose the heat it makes?

Correct answer
- Correct Only by evaporating sweat: the hot air actually adds heat to the skin
Setup, choices and feedback
The Scene at this step
Air temperature
298.1 Kfixed
Exercise
100 Wfixed
Thermostat
Workingfixed
Labels
offdefault
Zoom to the skin
Skin up close
Choices
- Correct Only by evaporating sweat: the hot air actually adds heat to the skin
- Wrong The cool sweat will cool the skin as it touches it
  Misconception: Thinks sweat cools the body because the liquid itself is cool, not because it evaporates.
  Feedback from the catalogue
  Sweat leaves the body at body temperature, so it is not cold. It cools by evaporating: every gram of water that turns to vapour takes about 2.4 kJ of heat from the skin. That is why sweat that drips off without evaporating, or sweat in very humid air, hardly cools at all.
- Wrong As usual, the skin will give heat to the air
  Feedback
  Heat flows from hot to cold. When the air is hotter than the skin, heat flows into the skin, not out of it.
Once the prediction is locked in, the Scene changes
Air temperature
313.1 Kfixed
The Scene then shows: 132.3 W · Heat carried off by evaporating sweat/nodes/person/outputs/evaporation
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Evaporation carries off about 130 W, and the air adds about 37 W to the skin. The body sweats about 0.19 L an hour, and thanks to that the core stays near 37 °C.

Worked solution
1. Heat flows from hot to cold. In 40 °C air the skin, at about 36 °C, gains heat from the air.
2. The only way left to lose heat is evaporation: every gram of sweat that evaporates takes about 2.4 kJ with it.
3. On the chart, evaporation is almost all the heat going out.
If you predicted that sweat cools because it is cool, it does not: it comes out warm, and cools only by evaporating.
Step 5 of 6
Open this step
The comfort zone
Practice · Reach the target
Question
Exercise and Thermostat are fixed. Change Air temperature until the body neither shivers nor sweats, and press Check.

Goal
- 0 W ± 1 W
  At the start of the step: 200 W
- 0 L/h ± 0.005 L/h
  At the start of the step: 0 L/h
Setup, choices and feedback
The Scene at this step
Air temperature
263.1 K
Exercise
100 Wfixed
Thermostat
Workingfixed
Labels
offdefault
Zoom to the skin
Person
Read from the Scene at
Heat made by shivering/nodes/person/outputs/shivering
Read from the Scene at
Sweat made, litres an hour/nodes/person/outputs/sweat
One setting that reaches it
Air temperature
297.1 K
Labels
off
Feedback for a right answer
In a narrow band around 298 K, about 25 °C, the body balances its heat by widening and narrowing the skin’s vessels alone, without shivering or sweating.

Feedback for a wrong answer
Not yet. If the body is shivering, warm the room; if it is sweating, cool it.

Hint
In a pleasant room the body manages with skin blood flow alone.

Worked solution
1. At 263 K the body is shivering flat out.
2. Warm the room step by step: the shivering weakens, and stops at around 297 K.
3. Go on, and sweating starts above about 299 K.
Between them, about 25 °C, skin blood flow alone balances the heat.
Step 6 of 6
Open this step
Negative feedback
Practice · Choose
Question
The body’s heat control is an example of negative feedback. What makes it negative feedback?

Correct answer
- Correct The response opposes the change: heat leads to cooling, and cold to warming
Setup, choices and feedback
The Scene at this step
Air temperature
298.1 K
Exercise
100 W
Thermostat
Working
Labels
offdefault
Zoom to the skin
Person
Choices
Shown in a random order.
- Correct The response opposes the change: heat leads to cooling, and cold to warming
- Wrong It harms the body, as shivering and sweating waste energy and water
  Misconception: Takes "negative" feedback to mean feedback that harms the body, or that makes a change bigger.
  Feedback from the catalogue
  In negative feedback the word “negative” describes the direction: the response opposes the change. When the temperature rises the body brings it down, and when it falls the body raises it, so it returns to its set point. It is a very useful mechanism: most of the body’s control is built from it. Feedback that enlarges a change is positive feedback, and it is rare, as in childbirth.
- Wrong The response enlarges the change until it stops
  Misconception: Takes "negative" feedback to mean feedback that harms the body, or that makes a change bigger.
  Feedback from the catalogue
  In negative feedback the word “negative” describes the direction: the response opposes the change. When the temperature rises the body brings it down, and when it falls the body raises it, so it returns to its set point. It is a very useful mechanism: most of the body’s control is built from it. Feedback that enlarges a change is positive feedback, and it is rare, as in childbirth.
Feedback for a right answer
Right. A sensor measures, a control centre, the hypothalamus, compares with the set point, and effectors, the vessels, sweat glands and muscles, push the value back. The same loop holds blood sugar, water and blood pressure.

Hint
Think about the direction of the response compared with the change.

Worked solution
1. There is a set point, about 37 °C.
2. Sensors in the skin and the brain detect a drift, and the hypothalamus starts a response.
3. The response opposes the drift: too hot, cool down; too cold, warm up.
“Negative” is the direction, not a judgement. Feedback that enlarges a change is called positive feedback.

Lesson 3 of 7 · About 25 minutes

Hormones

Chemical messengers in the blood: how insulin and glucagon hold blood sugar steady, when sugar peaks after a meal, where the sugar comes from when you skip a meal, and what happens without insulin.

Open the lesson

The 7 steps and their answers

Step 1 of 7
Open this step
Messengers in the blood
Practice · Choose
Question
Show the hormones is On. The pancreas sits in the belly, and the muscles that answer insulin are all over the body. How does insulin reach them?

Correct answer
- Correct In the blood: the pancreas releases it into the blood, which carries it everywhere
Setup, choices and feedback
The Scene at this step
Time since eating
30 min
Carbohydrate in the meal
50 g
Body
Healthy
Show the hormones
On
Choices
Shown in a random order.
- Correct In the blood: the pancreas releases it into the blood, which carries it everywhere
- Wrong Along nerves leading from the pancreas
  Misconception: Thinks hormones travel to their targets along nerves.
  Feedback from the catalogue
  Hormones do not travel along nerves. A gland releases them into the blood, and the blood carries them all over the body in about a minute. Only cells with a matching receptor respond. That is why hormones are slower than nerves, but their effect is wider and lasts longer.
- Wrong In special ducts from the pancreas to each muscle
  Feedback
  Hormone glands have no ducts: they release straight into the blood. The pancreas does send digestive juices along a duct, but only to the gut.
Feedback for a right answer
Right. The hormone reaches every cell in the body, but only cells with an insulin receptor respond. That is how one messenger can steer many organs at once.

Hint
Watch the specks flowing in the blood vessels.

Worked solution
1. Hormone glands, the endocrine system, release straight into the blood.
2. The blood reaches every organ within about a minute.
3. The cells that respond are those with a matching receptor.
Nerves carry signals fast and to one exact place; hormones more slowly, to the whole body and for longer.
Step 2 of 7
Open this step
Sugar in the blood
Practice · Calculate
Question
Before the meal blood sugar is 5.0 mmol/L. A millimole of glucose weighs 180 mg. How many milligrams of glucose are in a decilitre of blood, 0.1 L?

Correct answer
89.96 ± 2
Setup, choices and feedback
The Scene at this step
Time since eating
0 minfixed
Carbohydrate in the meal
50 gfixed
Body
Healthyfixed
Show the hormones
Off
Accepted within
± 2
Computed from the Scene at this step’s setting
Blood sugar, mg/dL/nodes/person/outputs/glucoseMg
Feedback for a right answer
$5.0 \times 180 = 900$ ⁠ milligrams per litre, and a tenth of that in a decilitre: 90 mg/dL. All the blood, about five litres, holds only about 4.5 g of sugar: less than a teaspoon.

Hint
First convert millimoles to milligrams per litre, then go from a litre to a decilitre.

Worked solution
1. Per litre: $5.0\ \mathrm{mmol} \times 180\ \mathrm{mg/mmol} = 900\ \mathrm{mg}$ ⁠.
2. Per decilitre, a tenth of a litre: $900 \div 10 = 90$ ⁠.
3. The answer: 90 mg/dL.
If you got 900, you gave the amount in a whole litre.
Step 3 of 7
Open this step
Insulin
Practice · Predict, then look
Question
A healthy person ate a meal with 50 g of carbohydrate. Which hormone will rise in the blood an hour after the meal?

Correct answer
- Correct Insulin
Setup, choices and feedback
The Scene at this step
Time since eating
0 minfixed
Carbohydrate in the meal
50 gfixed
Body
Healthyfixed
Show the hormones
On
Choices
- Correct Insulin
- Wrong Glucagon
  Misconception: Confuses glucagon, the hormone, with glycogen, the stored sugar.
  Feedback from the catalogue
  The names are alike, but the things are different. Glycogen is a store: long chains of glucose kept in the liver and the muscles. Glucagon is a hormone from the pancreas, a messenger, that tells the liver to break the glycogen down and release sugar into the blood when blood sugar falls. Insulin, the other hormone, tells the liver to store.
- Wrong Neither: the sugar will just go into the cells
  Feedback
  Most cells, in muscle and fat, take in hardly any glucose without insulin. The hormone is what opens the door.
Once the prediction is locked in, the Scene changes
Time since eating
60 minfixed
The Scene then shows: 56.53 · Insulin in the blood, µU/mL/nodes/person/outputs/insulin
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Insulin rises about sevenfold, from 8 to 57 units, and glucagon falls by more than half. Insulin tells the muscles to take sugar in and the liver to store it as glycogen.

Worked solution
1. Sugar from the gut raises the blood sugar.
2. Beta cells in the pancreas sense the rise and release insulin.
3. Insulin makes muscle and fat take in glucose and the liver store it, and the sugar falls again.
Glucagon works the other way: it rises when sugar falls.
Step 4 of 7
Open this step
The peak
Practice · Reach the target
Question
Carbohydrate in the meal and Body are fixed. Move Time since eating to the moment blood sugar is at its peak, and press Check.

Goal
- 6.91 mmol/L ± 0.025 mmol/L
  At the start of the step: 4.998 mmol/L
Setup, choices and feedback
The Scene at this step
Time since eating
0 min
Carbohydrate in the meal
50 gfixed
Body
Healthyfixed
Show the hormones
Off
Read from the Scene at
Blood sugar, mmol/L/nodes/person/outputs/glucose
One setting that reaches it
Time since eating
35 min
Feedback for a right answer
About 35–40 minutes after eating, at about 6.9 mmol/L. After the peak sugar is still being absorbed from the gut, but insulin is already clearing it from the blood faster.

Feedback for a wrong answer
Not yet. If the sugar is still rising, move later; if it is already falling, earlier.

Hint
Follow the sugar curve on the chart, and find its highest point.

Worked solution
1. Right after eating the sugar is about 5.0 mmol/L.
2. It rises while sugar from the gut arrives faster than insulin clears it.
3. The peak, about 6.9 mmol/L, comes at 35–40 minutes; after it insulin wins and the sugar falls.
By two hours it is almost back: negative feedback.
Step 5 of 7
Open this step
Glucagon
Practice · Choose
Question
What is glucagon?

Correct answer
- Correct A hormone from the pancreas that tells the liver to release sugar into the blood
Setup, choices and feedback
The Scene at this step
Time since eating
240 min
Carbohydrate in the meal
50 g
Body
Healthy
Show the hormones
On
Choices
Shown in a random order.
- Correct A hormone from the pancreas that tells the liver to release sugar into the blood
- Wrong The sugar the liver stores in long chains
  Misconception: Confuses glucagon, the hormone, with glycogen, the stored sugar.
  Feedback from the catalogue
  The names are alike, but the things are different. Glycogen is a store: long chains of glucose kept in the liver and the muscles. Glucagon is a hormone from the pancreas, a messenger, that tells the liver to break the glycogen down and release sugar into the blood when blood sugar falls. Insulin, the other hormone, tells the liver to store.
- Wrong A hormone that lowers blood sugar
  Feedback
  That is insulin. Glucagon works the other way: it raises blood sugar.
Feedback for a right answer
Right. Glucagon is the messenger, and glycogen is the store the messenger opens. The pancreas’s two hormones, insulin and glucagon, pull blood sugar in opposite directions.

Hint
Glucagon and glycogen are two different things, despite the names.

Worked solution
1. Glycogen: a store, chains of glucose in the liver and muscles.
2. Glucagon: a hormone from the alpha cells of the pancreas, released when blood sugar falls.
3. It tells the liver to break down glycogen and release glucose.
Step 6 of 7
Open this step
Between meals
Practice · Predict, then look
Question
Skip a meal: Carbohydrate in the meal at 0 g. What will blood sugar be four hours later?

Correct answer
- Correct Almost the same, about 5 mmol/L
Setup, choices and feedback
The Scene at this step
Time since eating
0 minfixed
Carbohydrate in the meal
0 gfixed
Body
Healthyfixed
Show the hormones
Off
Choices
- Correct Almost the same, about 5 mmol/L
- Wrong It will fall a lot, because no new sugar came in
  Misconception: Thinks blood sugar comes only from the last meal, so it drains away when a meal is skipped.
  Feedback from the catalogue
  Between meals blood sugar comes from the liver. When it starts to fall, the pancreas releases glucagon, and the liver breaks down glycogen and releases glucose, so blood sugar stays near 5 mmol/L hours after a meal, and right through the night.
Once the prediction is locked in, the Scene changes
Time since eating
240 minfixed
The Scene then shows: 4.998 · Blood sugar, mmol/L/nodes/person/outputs/glucose
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
5.0 mmol/L, just as at the start. The liver releases about 0.14 g of sugar a minute into the blood, and over the four hours its glycogen falls from about 50 g to about 28 g: the store is what holds the sugar up.

Worked solution
1. The brain and the cells keep using sugar even without a meal.
2. As the sugar starts to fall, the pancreas releases glucagon.
3. The liver breaks down glycogen and releases glucose, just as fast as it is used.
If you predicted a big fall, you thought the sugar comes only from food. Between meals it comes from the liver.
Step 7 of 7
Open this step
Without insulin
Practice · Predict, then look
Question
Two hours after the meal, in a healthy body, the sugar is almost back to 5 mmol/L. What will it be in a body whose pancreas makes no insulin at all, Type 1, no insulin?

Correct answer
- Correct Far above 10 mmol/L, and sugar will spill into the urine
Setup, choices and feedback
The Scene at this step
Time since eating
120 minfixed
Carbohydrate in the meal
50 gfixed
Body
Healthyfixed
Show the hormones
Off
Choices
- Correct Far above 10 mmol/L, and sugar will spill into the urine
- Wrong Almost normal, because glucagon will balance it
  Feedback
  Glucagon only raises sugar. Without insulin nothing brings it down, and the liver even keeps releasing more.
Once the prediction is locked in, the Scene changes
Body
Type 1, no insulinfixed
The Scene then shows: 20.32 · Blood sugar, mmol/L/nodes/person/outputs/glucose
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About 20 mmol/L, four times normal. Above about 10 mmol/L the kidneys cannot take all the sugar back, and it leaves in the urine. That is why type 1 diabetes needs insulin from outside.

Worked solution
1. Insulin is the only hormone that lowers blood sugar.
2. Without insulin the muscles take in hardly any sugar, and the liver does not stop releasing it.
3. The sugar reaches about 20 mmol/L, above the kidneys’ threshold, and spills into the urine.
Negative feedback needs its effector too: without insulin, the loop is broken.

Lesson 4 of 7 · About 25 minutes

The eye and vision

How the eye focuses: power in dioptres, the near point, the ageing eye, short sight and the far point, and which glasses correct each eye.

Open the lesson

The 7 steps and their answers

Step 1 of 7
Open this step
Focusing
Practice · Calculate
Question
A normal eye reads a card 0.25 m away. To focus on it the lens has to add a power of $\dfrac{1}{d}$ ⁠ dioptres, with $d$ ⁠ in metres. How many dioptres?

Correct answer
4 ± 0.1
Setup, choices and feedback
The Scene at this step
Card distance
0.25 mfixed
Eye
Normalfixed
Glasses
offfixed
Zoom to the retina
Eye
Accepted within
± 0.1
Computed from the Scene at this step’s setting
How hard the lens is focusing, dioptres/nodes/eye/outputs/accommodation
Feedback for a right answer
$\dfrac{1}{0.25} = 4$ ⁠: four dioptres. The closer the card, the more the lens has to round up.

Hint
$\dfrac{1}{0.25}$ ⁠.

Worked solution
1. The given: $d = 0.25$ ⁠ metres.
2. The power the lens adds: $\dfrac{1}{d} = \dfrac{1}{0.25} = 4$ ⁠.
3. The answer: 4 D.
For a very distant object $\dfrac{1}{d}$ ⁠ is almost zero, and the lens is relaxed. If you answered $0.25$ ⁠, you gave the distance, not the power.
Step 2 of 7
Open this step
The near point
Practice · Calculate
Question
The lens of a normal thirty-year-old eye can add up to 7 dioptres. What is the closest distance this eye sees sharply? Answer in centimetres.

Correct answer
14.29 cm ± 0.5 cm
Setup, choices and feedback
The Scene at this step
Card distance
0.3 m
Eye
Normalfixed
Glasses
offfixed
Zoom to the retina
Eye
Accepted within
± 0.5 cm
Computed from the Scene at this step’s setting
The closest distance this eye (with its glasses, if worn) sees sharply/nodes/eye/outputs/nearPoint
Any unit of the same quantity is accepted; it is converted before checking.
Feedback for a right answer
$\dfrac{1}{7} \approx 0.143$ ⁠ metres, about 14 cm. This is the eye’s near point. By sixty it has moved out to about 80 cm.

Hint
Turn the formula round: $d = \dfrac{1}{P}$ ⁠.

Worked solution
1. The most the lens can add: $P = 7$ ⁠ dioptres.
2. The distance: $d = \dfrac{1}{P} = \dfrac{1}{7} \approx 0.143$ ⁠ metres.
3. In centimetres: about 14.3 cm.
Any closer, the lens cannot round up enough, and the image blurs.
Step 3 of 7
Open this step
The ageing eye
Practice · Predict, then look
Question
A normal eye reads a card at 30 cm sharply. What will happen when Eye switches to Ageing, 60 years, whose lens has stiffened?

Correct answer
- Correct The card will blur: the lens cannot round up enough
Setup, choices and feedback
The Scene at this step
Card distance
0.3 mfixed
Eye
Normal
Glasses
offfixed
Zoom to the retina
Image on the retina
Choices
- Correct The card will blur: the lens cannot round up enough
- Wrong The card will stay sharp: the eye is still healthy
  Feedback
  The eye is healthy, but its lens has lost its spring. It adds only about 1.25 dioptres, and a card at 30 cm needs more than three.
Once the prediction is locked in, the Scene changes
Eye
Ageing, 60 years
The Scene then shows: 0.8 m · The closest distance this eye (with its glasses, if worn) sees sharply/nodes/eye/outputs/nearPoint
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The near point has moved out to 0.8 m, and the light from the card would come to a focus behind the retina. This is age-related long sight: at around forty-five people start holding books further away.

Worked solution
1. A card at 0.3 m needs $\dfrac{1}{0.3} \approx 3.3$ ⁠ dioptres.
2. The ageing lens adds only about 1.25 D.
3. It falls short, and the light would focus behind the retina: the card blurs.
Its near point: $\dfrac{1}{1.25} = 0.8$ ⁠ metres.
Step 4 of 7
Open this step
Short sight
Practice · Reach the target
Question
Eye is fixed at Short-sighted, without Glasses. The card is far, at 6 m, and blurred. Move Card distance until the eye sees it sharply, and press Check.

Goal
- 0 D ± 0.1 D
  At the start of the step: 2.729 D
Setup, choices and feedback
The Scene at this step
Card distance
6 m
Eye
Short-sightedfixed
Glasses
offfixed
Zoom to the retina
Image on the retina
Read from the Scene at
Focusing the eye could not supply, dioptres: positive when the light meets in front of the retina, negative behind it/nodes/eye/outputs/defocus
One setting that reaches it
Card distance
0.15 m
Feedback for a right answer
The card turns sharp only within about 35 cm. That is this eye’s far point: everything beyond it is blurred.

Feedback for a wrong answer
Not yet. Bring the card closer.

Hint
A short-sighted eye sees sharply close up.

Worked solution
1. The short-sighted eye is too long, so light from a distant object comes to a focus in front of the retina.
2. As the card comes closer, its light spreads more and the focus moves back, toward the retina.
3. At about 35 cm the focus lands on the retina, and closer still the lens makes up the rest.
That is why short-sighted people read without glasses, and wear them to see far.
Step 5 of 7
Open this step
Near or far
Practice · Choose
Question
A person is short-sighted. What do they see sharply without glasses?

Correct answer
- Correct Things close up, but not far away
Setup, choices and feedback
The Scene at this step
Card distance
0.3 m
Eye
Short-sighted
Glasses
off
Zoom to the retina
Image on the retina
Choices
Shown in a random order.
- Correct Things close up, but not far away
- Wrong Things far away, but not close up
  Misconception: Mixes up short sight and long sight: thinks a short-sighted person cannot see things close up.
  Feedback from the catalogue
  Short sight is named for sight that reaches only a short way: things near are sharp and things far away are blurred. The eye is too long for its power, so light from a distant object comes to a focus in front of the retina. In long sight the eye is too short, and light from a near object would come to a focus behind the retina.
- Wrong Nothing, near or far
  Feedback
  In the Scene’s short-sighted eye the card is sharp at any distance under about 35 cm.
Feedback for a right answer
Right. “Short sight”: sharp sight reaches only a short way. The opposite is long sight, in which near things blur.

Hint
Think of what you found in the last Step.

Worked solution
1. In a short-sighted eye light from a distant object focuses in front of the retina: far is blurred.
2. From a near object the light spreads more, and the focus lands on the retina: near is sharp.
3. So short sight is trouble seeing far.
If you chose “far”, you swapped short sight and long sight.
Step 6 of 7
Open this step
Glasses
Practice · Choose
Question
Which glasses correct the short-sighted eye? Switch on Glasses with the card at 6 m.

Correct answer
- Correct A diverging lens, of negative power, about −3 D
Setup, choices and feedback
The Scene at this step
Card distance
6 mfixed
Eye
Short-sightedfixed
Glasses
off
Zoom to the retina
Image on the retina
Choices
Shown in a random order.
- Correct A diverging lens, of negative power, about −3 D
- Wrong A converging lens, of positive power, about +3 D, that magnifies what you see
  Misconception: Thinks every pair of glasses works by magnifying, with a converging lens.
  Feedback from the catalogue
  Glasses do not magnify; they correct where the light comes to a focus. A short-sighted eye, which focuses too strongly, needs a diverging lens, a negative one, such as −3 D. A long-sighted or ageing eye, which focuses too weakly, needs a converging lens, a positive one.
Feedback for a right answer
Right. The diverging lens spreads the light a little before it enters the eye, as if it came from the eye’s far point, and the focus moves back onto the retina.

Hint
The short-sighted eye focuses too strongly. What would weaken its power?

Worked solution
1. The short-sighted eye focuses light in front of the retina: its power is too great for its length.
2. To weaken it, add negative power: a diverging lens.
3. The eye’s far point is about 0.35 m, so it needs about $-\dfrac{1}{0.35} \approx -3$ ⁠ dioptres.
A converging lens would add power and move the focus even further forward.
Step 7 of 7
Open this step
Reading glasses
Practice · Predict, then look
Question
The ageing eye reads sharply at 30 cm with reading Glasses of +2.5 D. What will happen when it looks through the same glasses at a card 6 m away?

Correct answer
- Correct The far card will be blurred
Setup, choices and feedback
The Scene at this step
Card distance
0.3 m
Eye
Ageing, 60 yearsfixed
Glasses
onfixed
Zoom to the retina
Image on the retina
Choices
- Correct The far card will be blurred
- Wrong The card will be sharp: the glasses fixed the eye
  Feedback
  The glasses do not change the eye itself. They add a fixed power that suits reading and gets in the way of distant sight.
Once the prediction is locked in, the Scene changes
Card distance
6 m
The Scene then shows: 2.4 · Focusing the eye could not supply, dioptres: positive when the light meets in front of the retina, negative behind it/nodes/eye/outputs/defocus
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Blurred: about 2.4 dioptres too much, and the light focuses in front of the retina. Reading glasses add power for one distance; that is why many people over forty-five wear bifocals.

Worked solution
1. For a distant object the ageing eye needs no extra power: without glasses it sees far sharply.
2. The glasses add +2.5 D regardless, and the lens cannot relax below its resting state.
3. The surplus moves the focus in front of the retina: blurred.
Glasses do not fix the eye; they correct its power for certain distances.

Lesson 5 of 7 · About 25 minutes

The brain at work

A folded sheet of neurons with a map of jobs: seeing at the back, movement in a strip across the top, crossed wiring so each side of the brain serves the opposite side of the body, speech that lives on the left, and neurons that fire faster when their area works.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
A folded sheet
Practice · Choose
Question
In which lobe is the visual cortex, where seeing starts in the brain?

Correct answer
- Correct The occipital lobe, at the very back
Setup, choices and feedback
The Scene at this step
Task
Seeing
Side
Right
Zoom to the neurons
Lobes
Choices
Shown in a random order.
- Correct The occipital lobe, at the very back
- Wrong The frontal lobe, close to the eyes
  Feedback
  Intuitive, but no. The eyes are wired by the optic nerves to the back of the brain, as far from them as it gets.
- Wrong The temporal lobe, above the ear
  Feedback
  That is where the hearing cortex is, in the deep groove above it.
- Wrong In the cerebellum
  Feedback
  The cerebellum smooths and times movements.
Feedback for a right answer
The occipital lobe, at the back of the head. From there signals flow forward two ways: down into the temporal lobe to recognise what a thing is, and up into the parietal lobe to know where it is and how to reach it. A blow to the back of the head can make you “see stars”.

Hint
Choose Seeing for Task and move Zoom to the neurons to Active areas.

Worked solution
1. At Lobes each lobe is tinted and named.
2. At Active areas with Seeing, the glow is at the very back.
3. The back is the occipital lobe: the visual cortex is there.
Step 2 of 6
Open this step
Crossed wiring
Practice · Predict, then look
Question
Moving the left hand lights the motor strip of the right hemisphere. Predict what glows when Side switches to Right.

Correct answer
- Correct The left motor strip, and the right half of the cerebellum
Setup, choices and feedback
The Scene at this step
Task
Moving a handfixed
Side
Left
Zoom to the neurons
Active areasfixed
Choices
- Correct The left motor strip, and the right half of the cerebellum
- Wrong The left motor strip, and the left half of the cerebellum
  Feedback
  The cortex is right, but the cerebellum is not crossed: it works with its own side of the body.
- Wrong The right motor strip, and the right half of the cerebellum
  Misconception: Thinks each half of the brain moves and feels the same side of the body.
  Feedback from the catalogue
  Most of the wiring between the cortex and the body is crossed. Fibres from the left motor cortex cross to the right in the brainstem and run down to the muscles of the right hand, and each hemisphere sees the opposite half of the view. That is why a stroke in the left hemisphere can paralyse the right side. The cerebellum is the exception: it works with its own side.
Once the prediction is locked in, the Scene changes
Side
Right
The Scene then shows: 0.6607 · Share of the activity above rest that is in the left half, 0..1/nodes/brain/outputs/leftShare
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The glow jumps to the top of the left hemisphere, and the cerebellum’s glow switches side the other way. Fibres from the motor cortex cross over low in the brainstem; the cerebellum works with its own side. About two-thirds of the extra activity is now on the left.

Worked solution
1. Left hand: the right motor strip. The wiring is crossed.
2. So the right hand: the left motor strip.
3. The cerebellum is the exception: right hand, its right half.
Step 3 of 6
Open this step
A busy patch
Practice · Predict, then look
Question
The patch in the left visual cortex is almost silent, because what is on your left is seen by the right hemisphere. A cortex neuron with nothing to do fires about once a second. Predict how many times a second, on average, the task’s neurons fire when Side switches to Right.

Correct answer
32.16 Hz ± 10 Hz
A number prediction counts as right within ± 10 Hz
Setup, choices and feedback
The Scene at this step
Task
Seeingfixed
Side
Left
Zoom to the neurons
Neuronsfixed
Once the prediction is locked in, the Scene changes
Side
Right
The Scene then shows: 32.16 Hz · Mean firing rate of the patch’s neurons that belong to the task/nodes/brain/outputs/taskRate
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About $32$ ⁠ times a second. In the raster the top rows fill with lines, while the rows below them, neurons outside the task, hardly change. A busy patch is not one where every neuron fires, but one where the right neurons fire dozens of times a second.

Worked solution
1. What is on your right reaches the left visual cortex: the patch wakes up.
2. The task’s neurons go from a few impulses a second to dozens.
3. Count the lines in one top row over the raster’s second: about $30$ ⁠.
Step 4 of 6
Open this step
Speech lives on the left
Practice · Predict, then look
Question
While speaking, about 62% of the extra activity is in the left hemisphere. Predict what happens when Side switches to Left.

Correct answer
- Correct Nothing: speech stays mostly on the left
Setup, choices and feedback
The Scene at this step
Task
Speakingfixed
Side
Right
Zoom to the neurons
Active areas
Choices
- Correct Nothing: speech stays mostly on the left
- Wrong Most of the activity moves to the right hemisphere
  Misconception: Thinks each half of the brain moves and feels the same side of the body.
  Feedback from the catalogue
  Most of the wiring between the cortex and the body is crossed. Fibres from the left motor cortex cross to the right in the brainstem and run down to the muscles of the right hand, and each hemisphere sees the opposite half of the view. That is why a stroke in the left hemisphere can paralyse the right side. The cerebellum is the exception: it works with its own side.
- Wrong The activity splits half and half
  Feedback
  Both sides of the mouth move, so both motor strips work. But Broca’s area, which plans the words, stays on the left.
Once the prediction is locked in, the Scene changes
Side
Left
The Scene then shows: 0.6201 · Share of the activity above rest that is in the left half, 0..1/nodes/brain/outputs/leftShare
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Nothing changes. Speech has no side: both sides of the lips and tongue move together, so both hemispheres’ motor strips work, but Broca’s area, which plans the words, is on the left in most people. Language is the clearest case of the two hemispheres not being the same.

Worked solution
1. Side picks the hand, the ear or the half of the view. Speech has no such side.
2. Broca’s area is in the left frontal lobe, in about 95% of right-handers.
3. So the split stays at about 62% on the left.
Step 5 of 6
Open this step
What a scan sees
Practice · Choose
Question
What does an fMRI scanner really measure when it shows a coloured patch in the hearing cortex?

Correct answer
- Correct Oxygen-rich blood flowing to the busy patch, compared with rest
Setup, choices and feedback
The Scene at this step
Task
Hearing
Side
Right
Zoom to the neurons
Active areas
Choices
Shown in a random order.
- Correct Oxygen-rich blood flowing to the busy patch, compared with rest
- Wrong The neurons’ electrical impulses, one by one
  Misconception: Thinks a brain scan such as fMRI shows the neurons firing directly.
  Feedback from the catalogue
  An fMRI scanner sees blood, not impulses. Working neurons need more oxygen, and within a few seconds the blood flow to a busy patch rises by more than they use, so the blood there is rich in oxygen. The scanner picks up that difference, and the picture is a difference between task and rest, a change of a few percent.
- Wrong New neurons growing in the area
  Feedback
  The task lasts seconds. No neurons grow in that time.
- Wrong The heat the whole brain gives off
  Feedback
  The brain’s total energy use hardly rises during a task. The scanner sees local differences in the blood.
Feedback for a right answer
Blood. Busy neurons need more oxygen, and the blood flow to the patch rises within a few seconds, by more than they use. The scanner picks up the difference between oxygen-rich and oxygen-poor blood. The patch is a difference of a few percent between task and rest, which is why the Scene shows it as a scan does: a map of what got busier.

Worked solution
1. Neurons that fire more use more oxygen.
2. The body responds with more blood flow to that patch.
3. Oxygen-rich and oxygen-poor blood behave differently in a magnetic field, and the scanner tells them apart.
4. So the patch is an indirect sign, a few seconds late, of the neurons’ activity.
Step 6 of 6
Open this step
Never idle
Practice · Choose
Question
At rest, with the mind wandering, most of the seeing and moving areas fade. Does that mean that most of the time we use only a small part of the brain, such as “10%”?

Correct answer
- Correct No: the whole brain is active all the time, and a task only makes some areas busier
Setup, choices and feedback
The Scene at this step
Task
Resting
Side
Right
Zoom to the neurons
Active areas
Choices
Shown in a random order.
- Correct No: the whole brain is active all the time, and a task only makes some areas busier
- Wrong Yes: most of the brain is kept in reserve
  Misconception: Believes we use only 10% of our brain.
  Feedback from the catalogue
  It is a myth. Every part of the brain is active all the time, even at rest and asleep; a task only makes some areas busier than they were. Damage almost anywhere in the brain harms something. And an organ that weighs 2% of the body and uses a fifth of its energy would not have survived evolution if nine-tenths of it were wasted.
- Wrong About half: only one hemisphere works at a time
  Feedback
  Almost every task uses both hemispheres, and even the resting default mode network spans both.
Feedback for a right answer
No. The glow in the Scene is a difference from rest, not a map of what is working at all. At rest the default mode network is actually busier, and neurons everywhere keep firing. The brain uses about a fifth of the body’s energy all the time, and that hardly rises when you think hard.

Worked solution
1. Every picture of “active areas” is task minus rest.
2. An area that does not glow is still active; it is just no busier than it was.
3. At rest, other areas glow: the default mode network.
4. So there is no dormant “90%”: the myth confuses “did not change” with “not working”.

Lesson 6 of 7 · About 25 minutes

Touch, warmth and pain

A fingertip as a laboratory of the senses: receptors that report strength by how fast they fire, fast and slow receptors, vibration that each receptor is tuned to a rhythm of its own, and separate nerve endings for cold, warmth and pain.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
One job
Practice · Reach the target
Question
At a Press of 0.8 mm a Merkel disc fires about $57$ ⁠ impulses a second while the press is held. Change Press until it fires $30$ ⁠ impulses a second, and press Check.

Goal
- 30 Hz ± 1.5 Hz
  At the start of the step: 57.27 Hz
Setup, choices and feedback
The Scene at this step
Press
0.8 mm
Vibration
0 Hzfixed
Probe temperature
306.1 Kfixed
Show the nerve signals
Nerve signals
Read from the Scene at
A Merkel disc's firing while the press is held, spikes a second/nodes/skin/outputs/merkelRate
One setting that reaches it
Press
0.2 mm
Feedback for a right answer
A press of 0.2 mm. The impulses themselves do not shrink when the press is light; each is the same all-or-nothing pulse. What changes is how many come each second. That is how every sense reports strength.

Feedback for a wrong answer
Not yet. If the disc fires too fast, press less; if too slowly, more.

Hint
A lighter press, fewer impulses. Count the lines in Merkel’s row on the chart.

Worked solution
1. A Merkel disc fires more as the skin is pressed deeper: from about $37$ ⁠ at 0.3 mm to about $66$ ⁠ at 1.5 mm.
2. $30$ ⁠ impulses a second is fewer than at 0.3 mm: press less than 0.3 mm.
3. At 0.2 mm the rate comes to $30$ ⁠.
Step 2 of 6
Open this step
Fast and slow
Practice · Choose
Question
You feel a shirt as you put it on, and a minute later you no longer do. What explains it, going by what the Scene shows?

Correct answer
- Correct Most touch receptors adapt: they fire while the skin moves, and fall silent or slow down when the press stays the same
Setup, choices and feedback
The Scene at this step
Press
0.8 mm
Vibration
0 Hz
Probe temperature
306.1 K
Show the nerve signals
Nerve signals
Choices
Shown in a random order.
- Correct Most touch receptors adapt: they fire while the skin moves, and fall silent or slow down when the press stays the same
- Wrong The receptors keep reporting at the same strength, and the brain just ignores it
  Misconception: Thinks a sense organ keeps reporting a steady stimulus at full strength for as long as it lasts.
  Feedback from the catalogue
  Most receptors mainly report change. Fast-adapting receptors, such as Meissner and Pacinian corpuscles, fire while the skin moves and fall silent when it stops; even slow-adapting ones, such as Merkel discs, fire hard at first and then settle. That is why you stop feeling your clothes, and why a hot bath feels hotter at first.
- Wrong The receptors tire out and die
  Feedback
  They are not harmed: move the shirt, and they fire again at once.
- Wrong The shirt stops pressing on the skin
  Feedback
  The shirt presses just as much; it only stops moving.
Feedback for a right answer
Adaptation. Meissner and Pacinian corpuscles adapt fast: they flash only as the probe goes in and comes out. Merkel discs adapt slowly, firing hard at first and then settling. The system is built to report change, because change is what needs attention: a thing starting to slip from the hand, an insect landing on the skin.

Worked solution
1. On the chart, in one press: Meissner and Pacinian fire only at its start and end.
2. Merkel fires all through the press, harder at the start.
3. A shirt that does not move is a steady press: most fibres are silent or slower, and little reaches the brain.
Step 3 of 6
Open this step
Tuned to a rhythm
Practice · Predict, then look
Question
At 30 Hz, a flutter, the Meissner corpuscles fire an impulse for each tap. Predict which receptor keeps time when Vibration rises to 250 Hz, a buzz like a phone buzzing in a pocket.

Correct answer
- Correct Only the Pacinian corpuscles, deep in the fat
Setup, choices and feedback
The Scene at this step
Press
0.8 mmfixed
Vibration
30 Hz
Probe temperature
306.1 Kfixed
Show the nerve signals
Nerve signals
Choices
- Correct Only the Pacinian corpuscles, deep in the fat
- Wrong The Meissner corpuscles, faster
  Feedback
  Meissner corpuscles are tuned to slow flutter. At 250 Hz they do not respond at all.
- Wrong The Merkel discs
  Feedback
  The Merkel discs keep reporting the press, but do not keep time with a vibration.
- Wrong All of them, each faster
  No feedback of its own; the learner sees the standard message
  Not yet. Look again and try once more.
Once the prediction is locked in, the Scene changes
Vibration
250 Hz
The Scene then shows: 250 Hz · A Pacinian corpuscle's firing while the press is held/nodes/skin/outputs/pacinianRate
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Only the Pacinian, an impulse each cycle, $250$ ⁠ a second. The layers wrapped round it like an onion filter out a slow press and pass only fast change, and at this rhythm the skin need move less than a micrometre for it to fire. The Meissner corpuscles have gone quiet. Each receptor is tuned to its own rhythm.

Worked solution
1. Meissner: most sensitive around 30 Hz.
2. Pacinian: most sensitive around 250 Hz.
3. At 250 Hz only the Pacinian pass threshold, and fire once each cycle: 250 Hz.
4. At 80 Hz, a hum in between, both kinds fire.
Step 4 of 6
Open this step
Warm endings
Practice · Reach the target
Question
At skin temperature the warm endings fire slowly. Change Probe temperature until a warm ending fires as fast as it can, and press Check.

Goal
- 30 Hz ± 0.5 Hz
  At the start of the step: 0.9091 Hz
Setup, choices and feedback
The Scene at this step
Press
0.8 mmfixed
Vibration
0 Hzfixed
Probe temperature
306.1 K
Show the nerve signals
Nerve signals
Read from the Scene at
A warm ending's firing while the probe touches/nodes/skin/outputs/warmRate
One setting that reaches it
Probe temperature
316.1 K
Feedback for a right answer
At 43 °C, about $30$ ⁠ impulses a second. Above that the warm endings actually slow down, and right there the pain endings start firing. A warm ending alone cannot tell 40 °C from 47 °C: at both it fires about $19$ ⁠ times a second.

Feedback for a wrong answer
Not yet. It is not “as hot as possible”: find the peak of the rate.

Hint
Warm a little at a time, and follow the warm endings’ row on the chart. If the rate starts to fall, come back a little.

Worked solution
1. Warm endings start firing above about 32 °C.
2. The rate climbs with warmth: $19$ ⁠ at 40 °C, $25$ ⁠ at 42 °C.
3. At 43 °C it peaks, $30$ ⁠; at 45 °C it is already $26$ ⁠.
Step 5 of 6
Open this step
Too hot
Practice · Predict, then look
Question
At 40 °C, warm, the warm endings fire about $19$ ⁠ times a second. Predict what happens when Probe temperature rises to 48 °C, hot.

Correct answer
- Correct The warm endings slow down, and the pain endings start firing
Setup, choices and feedback
The Scene at this step
Press
0.8 mmfixed
Vibration
0 Hzfixed
Probe temperature
313.1 K
Show the nerve signals
Nerve signals
Choices
- Correct The warm endings slow down, and the pain endings start firing
- Wrong The warm endings fire much faster, and that is what feels like pain
  Misconception: Thinks the skin has one kind of temperature sensor that simply fires more the hotter it gets.
  Feedback from the catalogue
  The skin has several separate kinds of nerve ending: cold endings, which fire faster as it cools below skin temperature, warm endings, which fire more from about 32 °C up to about 43 °C and then slow down, and pain endings, which join in at extreme heat and cold. The brain knows “too hot” from the pain endings firing, not from the warm endings firing faster.
- Wrong The cold endings fire, which is why very hot sometimes feels cold
  Feedback
  The cold endings stay silent while the probe is warmer than the skin.
Once the prediction is locked in, the Scene changes
Probe temperature
321.1 K
The Scene then shows: 24.55 Hz · A pain ending's firing while the probe touches/nodes/skin/outputs/painRate
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The warm endings slow to about $14$ ⁠ impulses a second, and the pain endings, which were silent, fire about $25$ ⁠ times a second. “Too hot” is a separate signal, in separate fibres. Their impulses crawl along thin, bare C fibres at about a metre a second, far behind the touch impulses.

Worked solution
1. The warm endings fire fastest at 43 °C, and slow above it.
2. TRPV1 in the pain endings opens above about 43 °C: at 48 °C they fire.
3. The brain knows pain from the pain endings’ fibres, not from a fast rate in the warm endings.
Step 6 of 6
Open this step
Mint and chilli
Practice · Choose
Question
Why does a mint feel cool in the mouth, though it is at room temperature?

Correct answer
- Correct Menthol opens TRPM8 in the cold endings, and the fibre reports “cold” to the brain
Setup, choices and feedback
The Scene at this step
Press
0.8 mm
Vibration
0 Hz
Probe temperature
293.1 K
Show the nerve signals
Nerve signals
Choices
Shown in a random order.
- Correct Menthol opens TRPM8 in the cold endings, and the fibre reports “cold” to the brain
- Wrong Menthol evaporates fast and cools the mouth
  Misconception: Thinks mint really cools the skin and chilli really heats it.
  Feedback from the catalogue
  Mint and chilli do not change the temperature. Menthol binds to the TRPM8 channel in cold endings and opens it, and capsaicin binds to TRPV1 in the pain endings that respond to heat. A nerve fibre can report only what it always reports, and the brain reads the impulses by the fibre: “cold”, or “hot and painful”.
- Wrong It is a taste, which the tongue reads as cold
  Feedback
  Taste cells detect sweet, salty, sour, bitter and umami. The coolness comes from temperature nerve endings.
- Wrong Menthol sets off the pain endings
  Feedback
  That is what capsaicin does, in chilli, through TRPV1. Menthol works on the cold endings.
Feedback for a right answer
Menthol binds to TRPM8 and opens it, just as cold does. A cold ending’s fibre can report only one thing, and the brain reads the impulses by the fibre: “cold”. In the same way capsaicin opens TRPV1, and chilli “burns” with no heat at all. David Julius shared the $2021$ ⁠ Nobel Prize for finding these channels.

Worked solution
1. A cold ending fires when its TRPM8 channel opens.
2. Cold usually opens it, below about 26 °C; menthol opens it too.
3. A nerve fibre reports by what it is, not by what set it off.
4. So the brain gets “cold” though the temperature has not changed.

Lesson 7 of 7 · About 25 minutes

Pitch and place

How the ear hears: tiny pushes of air, a wave that sorts pitch along the cochlea, a quiet sound heard only at some pitches, and high notes that go first with age and noise.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
Tiny pushes
Practice · Calculate
Question
Ordinary conversation is 60 dB. By how much does the air pressure at the eardrum swing? ( $0$ ⁠ dB is 20 µPa, and every 20 dB is $10$ ⁠ times the pressure.)

Correct answer
0.02 Pa ± 3%
Setup, choices and feedback
The Scene at this step
Pitch (powers of ten)
3 log Hz
Loudness
60 dBfixed
Listener's age
20 years
Uncoil the cochlea
Ear
Accepted within
± 3%
Computed from the Scene at this step’s setting
The sound’s pressure swing at the ear (RMS), 20 µPa times 10^(dB/20)/nodes/ear/outputs/pressure
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 20 Pa ± 5%
  Misconception: Multiplies the sound pressure by ten for every 10 dB, as for the energy, instead of every 20 dB.
  Feedback from the catalogue
  Every 10 dB is $10$ ⁠ times the sound’s energy, but the energy grows as the square of the pressure. So the pressure grows $10$ ⁠ times only every 20 dB: $p = 20\ \mu\text{Pa} \times 10^{L/20}$ ⁠. A conversation, 60 dB, is $10^3$ ⁠ times 20 µPa, not $10^6$ ⁠.
Feedback for a right answer
0.02 Pa, $1000$ ⁠ times 20 µPa. The air’s own pressure is about 101,000 Pa, so conversation rocks it by less than two parts in a million. In a whisper the eardrum moves less than the width of an atom. The ear is one of the most sensitive sensors in the body.

Hint
How many times does 20 dB go into 60 dB?

Worked solution
1. $\dfrac{60}{20} = 3$ ⁠: three times $10$ ⁠, so $10^3 = 1000$ ⁠ times.
2. $20\ \mu\text{Pa} \times 1000 = 20\,000\ \mu\text{Pa}$ ⁠.
3. 0.02 Pa.
If you got 20 Pa, you multiplied by $10$ ⁠ every 10 dB: that is the rule for energy, not pressure.
Step 2 of 6
Open this step
A wave that sorts
Practice · Predict, then look
Question
A 1 kHz tone peaks about 21 mm from the base. Predict where a Low hum 50 Hz peaks.

Correct answer
- Correct Nearly at the tip, far from the base
Setup, choices and feedback
The Scene at this step
Pitch (powers of ten)
3 log Hz
Loudness
60 dB
Listener's age
20 years
Uncoil the cochlea
Cochlea uncoiledfixed
Choices
- Correct Nearly at the tip, far from the base
- Wrong Near the base, by the oval window
  Misconception: Thinks high pitches peak at the far end of the cochlea and low pitches near its start.
  Feedback from the catalogue
  The other way round. The basilar membrane is narrow and stiff at the base, by the oval window, and there it vibrates to high pitches. Towards the tip it is wide and floppy, and that is where low pitches peak. Every wave sets out from the base, and a low tone travels nearly to the end before it peaks.
- Wrong In the same place; only the wave is slower
  Feedback
  Each pitch peaks at a place of its own: that is what lets the brain tell them apart.
Once the prediction is locked in, the Scene changes
Pitch (powers of ten)
1.7 log Hz
The Scene then shows: 33.78 mm · Where the tone peaks on the basilar membrane, measured from the base/nodes/ear/outputs/place
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About 34 mm from the base, nearly at the 35 mm tip. The tip is wide and floppy, and there the membrane vibrates to low pitches. The hum’s wave travels along almost the whole membrane, slowing and growing, and only there does it peak.

Worked solution
1. A narrow, stiff base: high pitches. A wide, floppy tip: low pitches.
2. 50 Hz is far lower than 1 kHz: the peak moves towards the tip.
3. On the scale along the uncoiled cochlea: about 34 mm.
Step 3 of 6
Open this step
One place, one pitch
Practice · Reach the target
Question
Change Pitch (powers of ten) until the wave peaks 10 mm from the base, and press Check.

Goal
- 10 mm ± 0.5 mm
  At the start of the step: 20.99 mm
Setup, choices and feedback
The Scene at this step
Pitch (powers of ten)
3 log Hz
Loudness
60 dBfixed
Listener's age
20 yearsfixed
Uncoil the cochlea
Cochlea uncoiled
Read from the Scene at
Where the tone peaks on the basilar membrane, measured from the base/nodes/ear/outputs/place
One setting that reaches it
Pitch (powers of ten)
3.68 log Hz
Feedback for a right answer
About 5 kHz, about $3.7$ ⁠ on the slider. A higher tone peaks closer to the base. From 1 kHz to 5 kHz the peak moves about 11 mm; each doubling of pitch moves it a few millimetres.

Feedback for a wrong answer
Not yet. If the peak is too far from the base, raise the pitch; if too close, lower it.

Hint
10 mm is closer to the base than 21 mm. Which way must the pitch go?

Worked solution
1. At 1 kHz the peak is about 21 mm from the base.
2. You need a peak closer to the base: a higher tone.
3. Move the slider up and read the place on the scale until it says 10 mm: about $3.7$ ⁠, so $10^{3.7} \approx 5000$ ⁠ Hz.
Step 4 of 6
Open this step
Quiet and loud
Practice · Predict, then look
Question
A whisper, 30 dB, is easily heard at speech pitches. Predict whether a 20-year-old hears a Low hum 50 Hz when Loudness drops to Whisper.

Correct answer
- Correct No: at this pitch 30 dB is below the threshold of hearing
Setup, choices and feedback
The Scene at this step
Pitch (powers of ten)
1.7 log Hzfixed
Loudness
60 dB
Listener's age
20 yearsfixed
Uncoil the cochlea
Cochlea uncoiled
Choices
- Correct No: at this pitch 30 dB is below the threshold of hearing
- Wrong Yes: 30 dB is 30 dB at any pitch
  Misconception: Thinks a sound of a given number of decibels is equally easy to hear at every pitch.
  Feedback from the catalogue
  The threshold of hearing depends on pitch. The ear is most sensitive between 2 kHz and 5 kHz, where you hear even below 0 dB; a 50 Hz hum needs more than 40 dB just to be heard. The same number of decibels can be clear at one pitch and inaudible at another.
- Wrong Yes, and even louder than a whisper, since low sounds carry far
  Feedback
  Low sounds do carry far in air, but the ear is less sensitive to them.
Once the prediction is locked in, the Scene changes
Loudness
30 dB
The Scene then shows: -13.95 · How far the tone is above the listener’s threshold, dB (negative: not heard)/nodes/ear/outputs/sensation
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Not heard. At 50 Hz the threshold of hearing is about 44 dB, and 30 dB is about 14 dB below it. The wave reaches the tip, but no hair cell moves enough to fire. At the same level a 3 kHz tone would be clearly heard.

Worked solution
1. The threshold depends on pitch: lowest between 2 kHz and 5 kHz, high at the ends.
2. At 50 Hz the threshold is about 44 dB.
3. $30 - 44 = -14$ ⁠ dB: below the threshold, not heard.
Step 5 of 6
Open this step
High notes go first
Practice · Predict, then look
Question
A 20-year-old just hears a Mosquito tone 17 kHz at 60 dB. Predict what happens when Listener’s age goes up to 45 years.

Correct answer
- Correct The tone is no longer heard at all
Setup, choices and feedback
The Scene at this step
Pitch (powers of ten)
4.23 log Hzfixed
Loudness
60 dBfixed
Listener's age
20 years
Uncoil the cochlea
Cochlea uncoiled
Choices
- Correct The tone is no longer heard at all
- Wrong It sounds a little quieter, like every other tone at that age
  Misconception: Thinks hearing fades with age equally at every pitch, like turning the volume down.
  Feedback from the catalogue
  Hearing usually fades from the top down. Every tone passes the base of the cochlea, where the high-pitch hair cells sit, so they wear out first and do not grow back. At $45$ ⁠ a 17 kHz tone is already gone, while speech at 1 kHz has hardly changed.
- Wrong Nothing: at $45$ ⁠ hearing is still complete
  Feedback
  Nearly true for low and middle pitches. Not for the highest.
Once the prediction is locked in, the Scene changes
Listener's age
45 years
The Scene then shows: -35.29 · How far the tone is above the listener’s threshold, dB (negative: not heard)/nodes/ear/outputs/sensation
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Gone. At $45$ ⁠ the threshold at 17 kHz rises to about 95 dB, and 60 dB is far below it. Many hair cells are missing at the base of the membrane. Every tone passes there on its way, so they wear out first. That is why “mosquito” devices can drive teenagers away without bothering adults.

Worked solution
1. At $20$ ⁠: the threshold at 17 kHz is about 57 dB, and the tone about 3 dB above it.
2. Age raises the threshold most at high pitches: at 17 kHz by about 38 dB by $45$ ⁠.
3. $60 - 95 \approx -35$ ⁠ dB: not heard.
Step 6 of 6
Open this step
Noise
Practice · Choose
Question
After a whole evening at a rock concert, 105 dB, your ears ring. What happens to the hair cells the noise killed?

Correct answer
- Correct They are never replaced, and the loss adds up
Setup, choices and feedback
The Scene at this step
Pitch (powers of ten)
3.48 log Hz
Loudness
105 dB
Listener's age
20 years
Uncoil the cochlea
Cochlea uncoiled
Choices
Shown in a random order.
- Correct They are never replaced, and the loss adds up
- Wrong They grow back within a few days, when the ringing stops
  Misconception: Thinks hair cells damaged by noise recover or grow back, so ringing ears always heal.
  Feedback from the catalogue
  In people and other mammals, hair cells that die are not replaced. Ringing after a concert can fade, but every long exposure to noise above about 85 dB kills more cells, and the loss adds up for life. That is why hearing is protected in advance, with earplugs and lower volume.
- Wrong Only the low-pitch cells are hurt, since they vibrate hardest
  Feedback
  Every sound passes the base, so noise harms the high-pitch cells first.
Feedback for a right answer
In mammals, hair cells that die are not replaced. The ringing can fade, because damaged cells sometimes recover, but the dead ones are gone, and hours above 85 dB kill more. So noise speeds up what age does anyway, from the high pitches down. Birds and fish do regrow hair cells, and researchers are trying to learn how.

Worked solution
1. 105 dB is far above 85 dB, the level at which hours of noise do harm.
2. The hair cells’ bristles are bent too hard, and cells die.
3. Mammals have no source of new hair cells: the loss is permanent.
4. So protect in advance: earplugs, distance from the speakers, lower volume.

Across the unit · 23 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

Nerve signals

A cell at rest

Question

Correct answer

The Scene at this step

Hint

Worked solution

The threshold

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

All or nothing

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Strength as rate

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

What travels along the axon

Question

Correct answer

The Scene at this step

Choices

Hint

Worked solution

How fast

Question

Correct answer

The Scene at this step

Hint

Worked solution

Without myelin

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Homeostasis and feedback

Thirty-seven degrees

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

A freezing night

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

The thermostat

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Hotter than the skin

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

The comfort zone

Question

Goal