Any action that does not happen naturally or spontaneously, such as lifting something heavy from a low place to a high place or moving heat from a cold place to a hot place, requires energy. Because almost none of the human activities of “making things” and “daily life” occur spontaneously, they nearly all require energy. Therefore, energy is an essential piece of the puzzle in figuring out how to sustain the biosphere while we provide a modern standard of living for the human population of the earth. But many fundamental concepts of energy are difficult to grasp. Although a lesson on what energy is and what it means to consume energy may seem unexpected in a book about creating a sustainable society, it is important that we clarify these concepts before introducing ways in which technology can be used to make human existence on the earth sustainable.