You've probably heard lots of things about energy. You may know that your body needs energy or that there's energy in sunlight. But what exactly is energy? Where does it come from and where does it go? How do plants, people, and machines use energy? Scientists have wondered about these things too. They conducted experiments to learn the answers to their questions. How do cells get energy from food? What is nuclear energy? How does a roller coaster move? Find out what scientists have discovered about energy and how we use different forms of energy every day.