To begin to answer this deceptively simple question, come visit this site—and pick up a good working knowledge of elementary physics at the same time. Animations and interactive Java applets help illustrate important concepts, including a small-scale model of heat-transfer physics. In “The Heat Pump,” the physics behind heat pumps, refrigerators, and air conditioners is explained through animated graphics and detailed description. Another hands-on experiment is “The Rubber Band,” which reiterates the laws of physics explained by the heat pump through a simple demonstration that kids can try themselves.