Heat, the energy possessed by molecules because of their motion. Since all solids, liquids, and gases are made of molecules that move about, all such substances contain heat. Throughout the universe, heat energy is constantly being transferred from one substance or body to another and is being generated or expanded in all kinds of energy transformations. Heat has some part in every physical occurrence, from the explosion of a star to the growth of a blade of grass.

All living things need moderate amounts of heat. Too much or too little heat can destroy life. Heat brings about changes in the weather, by evaporating water to form rain clouds and causing the winds to circulate. Humans have learned to control heat energy for their own purposes. Heat is used to prepare food, make houses comfortable in winter, run engines, and carry out industrial processes.