Energy is the capacity for doing work. In physics, "work" has a more formal definition than in everyday life: it means the ability to exert a force through a distance. If you pick up this book, energy stored in molecular bonds inside your body is released to move the book's mass. The energy was stored in the molecules of the foods you ate and is released through a chemical reaction. Food provides the fuel that gives us energy.

Similarly, whether we are talking about automobile engines or power plant boilers, we need to have a fuel with stored energy that can be released in a useable way. Fossil fuels such as coal, oil, and natural gas provide much of the energy we use in industry and in our personal lives. These fuels were created by geological processes over millions of years, as plants and marine microorganisms consisting largely...