100 atoms that attract each other at short range, like argon, simulated live. Cold, each atom only jiggles about a fixed place in a crystal: a solid. Warm it and the jiggling grows until atoms break out of their places and slide past each other while still touching: a liquid. Hotter still, they move fast enough to escape the attraction completely and fly about the box: a gas. Red atoms are moving fast enough to break away from a neighbour.