It is not so difficult to imagine what it means to feed on the organic molecules that living things produce. That is, after all, what we do. But what does it mean to “feed” on sunlight, as phototrophs do? Or, even stranger, to “feed” on rocks, as lithotrophs do? Where is the “food,” for example, in the mixture of chemicals (H₂S, H₂, CO, Mn⁺, Fe²⁺, Ni²⁺, CH₄, and NH⁴⁺) that spews from a hydrothermal vent?