There are four phases or states of matter: solid, liquid, gas, and plasma. Solids have a definite shape and volume, liquids have an indefinite shape but definite volume, gases have an indefinite shape and volume, and plasma is made up of charged particles. Phase changes between these states require energy and are physical changes rather than chemical changes. Most matter changes from solid to liquid to gas as it absorbs more energy, though some substances like dry ice sublimate directly from solid to gas.