Molecules move faster (and atoms of a molecule vibrate and move apart) if heat is added. A good example that we are all familiar with is water. Water rains down on you in the shower but hot water also creates a lot of steam- the atoms in the water molecule move apart and the water molecule gets lighter- rising up and over the shower and fogging up the mirror. The opposite happens when water gets colder, the atoms get closer together and water molecules slow down then eventually get stuck in a structure (or freeze) which we see as ice.