Some types of matter, such as water, exist in more than one state. Heat energy causes matter to change from a solid to liquid, to a gas. This energy will make the particles of matter move faster or slower. The particles of a solid are fixed. Liquid particles form groups that move past one another. The groups of gas particles are completely broken. If enough energy is added, a solid may melt and form a liquid. Liquids can become gases if enough heat energy is added.