What actually changes?
An ice cube melts into a puddle. The puddle evaporates. Through it all, each water molecule is still H₂O: two hydrogen atoms joined to one oxygen atom. What changes is how the molecules are arranged and how they move around one another.
In ice, molecules vibrate around positions in a crystal structure. In liquid water, they continually move past their neighbors while remaining close together. In water vapor, they are much farther apart and move freely between collisions.
The scene uses a simple grid to represent a solid. Real ice has a more open crystal structure, which helps explain why ice is less dense than liquid water and floats. Other solids, such as glass, do not have a regular repeating crystal structure at all.
Follow the energy
Heating transfers energy into a substance. Within a single state, a higher temperature generally means more vigorous molecular motion. Cooling transfers energy out, reducing that motion.
At a change of state, something different happens: energy can change the arrangement of the molecules without raising the temperature. Energy added to melting ice helps loosen its structure. During boiling, energy separates molecules into the gas phase. This energy is called latent heat.
The temperature slider directly controls the illustration: moving it changes the sample immediately. Watch the ice shrink as meltwater gathers, then choose Boiling to see bubbles rise and burst before moving on to Gas. To make the changes explorable, melting and vaporization are spread across short slider ranges. Those ranges are illustrative, not physical temperature intervals for a phase change; the scene does not calculate heat input, latent heat, or elapsed time. Cooling runs the illustration backward, so the puddle gathers into a cube again. Real meltwater would refreeze as a flat sheet in the shape of its puddle.
Try it: choose Solid, Liquid, Boiling, then Gas. Choose Molecules under Zoom to the molecules and compare how much room the particles have to move.
The way back is just as interesting
At standard atmospheric pressure, pure water melts at about 0 °C (273 K) and boils at about 100 °C (373 K). Cool water vapor and it can condense into liquid. Cool that liquid enough and it freezes into ice. The changes run in both directions.
Boiling is not the only way water becomes gas. Evaporation happens at the surface even below the boiling point — that is how a puddle dries on a mild day. The boiling point also changes with pressure: water boils at a lower temperature high in the mountains.
Water vapor itself is invisible. The white cloud above a kettle consists of tiny liquid droplets formed when vapor cools and condenses. The scene’s cloud is a visual stand-in for the gas, so you can follow it.
Beyond these three
Solid, liquid, and gas are the familiar starting points. Plasma is another state: a gas in which some or many electrons have separated from atoms, creating a mixture of charged particles. It occurs in lightning and stars.
The bigger lesson remains the same: the properties we see at everyday scales emerge from what tiny particles do together.