Energy is one of the most important unifying themes in science. Yet the way energy is conceptualized varies depending on context. Asking students "What is energy?" generally yields textbook definitions such as "energy is the ability to do work." Rote responses such as these tell instructors little about how students understand energy. Here, student-generated analogies are shown to be an effective means of eliciting ideas about energy in systems from biology, chemistry, and physics. Six substance metaphors for energy were identified in the student responses: energy as a substance that can be accounted for; can flow; can be carried; can change forms; can be lost; and can be an ingredient, a product, or stored in some way. Each of these conceptual metaphors highlight and obscure various characteristics of energy and provide a set of frameworks that each afford a different understanding of the energy concept. (Contains 2 tables and 3 figures.)