Answer: Matter changes state when energy is added or taken away. Most matter changes because of heat energy. Changes in pressure can change matter, too.The atoms and molecules that make up matter are always moving. Even the molecules in solids are moving, just not very much. When matter is heated enough, the molecules move faster and with greater energy. If enough heat is added, a solid can become liquid and a liquid can become gas. In the same way, molecules slow down when they are cooled. Matter that is cooled to a certain point changes. For example, a liquid becomes a solid.