The signs of life.

Step 1 of 7

Seven signs

A mouse is alive, a stone is not. Every child knows that. But try to put the difference into words and it turns out to be hard: wind moves too, a crystal grows, and fire “eats” and “breathes”. Biologists do not settle for one sign. A living thing shows seven signs, all together:

1. Cells: it is made of one cell or more.
2. Energy: it breaks down food and uses the energy in it (metabolism).
3. Growth: it grows and develops by its own instructions.
4. Response: it reacts to stimuli from its surroundings.
5. Reproduction: it makes offspring.
6. Homeostasis: it keeps its inside steady, even when the outside changes.
7. Evolution: its offspring inherit its traits, and groups of living things change over generations.

The turntable holds six specimens, each with a placard of seven lamps. The mouse is at the front, and Look inside is open as far as Evolution: all seven of its lamps light blue. Blue means “has the sign”, amber “only looks like it”, and red “does not”. Change Specimen and look at every placard.

Step 2 of 7

The candle flame

A flame is the hardest case. It moves, grows, takes in “food” and oxygen and gives out carbon dioxide and water, just like breathing. It even “reproduces”: you can light another candle from it.

Before you open Look inside, decide for yourself.

Step 3 of 7

Imitations

All the flame’s lamps are lit now. Some are amber: signs the flame imitates without really having them. Its burning is a chemical reaction nobody runs; its “growth” is only more wax burning; and its “offspring” inherit nothing from it.

Step 4 of 7

The virus

The coronavirus, magnified 300,000 times. Move Time watched up to a week: it does not change. On its own it does not feed, grow or respond. But inside a living cell it makes thousands of copies of itself, and the copies change over generations.

Step 5 of 7

On the border

A whole day has passed, and the virus is exactly as it was. It is neither alive nor wholly non-living: biologists place it on the border.

Step 6 of 7

One bacterium becomes many

One E. coli bacterium lies on nutrient jelly at body temperature. In these conditions it divides every 20 min: each cell becomes two cells, and each of those two again. That is reproduction, and it is exactly where the bacterium differs from the crystal and the flame: the new cells are built from the mother cell’s DNA.

Two hours have passed since the start. Count.

Step 7 of 7

A colony

As time passes, the globe at the front zooms out so the colony fits inside it. A colony of bacteria becomes visible to the eye at about 0.2 mm across, and that happens within a few hours. Time watched is free now.