Energy, potential energy, is stored in the covalent bonds holding atoms together in the form of molecules. This is often called chemical energy. Except at absolute zero (the coldest temperature it is possible to reach), all molecules move. This movement is a form of kinetic energy, and the more the molecules move the more kinetic energy they have. Molecules in solids don't move much, they just vibrate. Molecules in liquids move faster and further, but they stick together enough to hold them in a small volume - the liquid. In a gas, however, the molecules are moving fast (over 1,000 miles per hour), moving in all directions, and don't stay together so they spread out and fill the available volume.