Energy comes in many different forms. It can be stored in the chemical bonds that hold molecules together, carried in the motion of a spinning wheel, or held in a boulder sitting on a cliff. But energy isn't a fixed thing. In fact, we convert it from one form to another all the time. People discovered energy conversion thousands of years ago. While they couldn't possibly have known what they were doing, when early humans burned wood to stay warm and keep predators away, they were actually converting the chemical energy stored in the wood into the heat and light of fire.