From the grass beneath our feet to overhead powerlines, surging with electrons, to distant galactic clusters, our universe is made of matter and its interplay with myriad forms of energy. But what exactly are these things? We know that energy--manifested as heat, light, electricity, magnetism, or sound--can cause matter to move, heat up, expand, glow, and undergo many sorts of interactions. And we know that matter can take a nearly infinite variety...