Mass is not what it seems. This is because we inhabit a world of weight, density, texture, and tangibility. The realities produced by calculus and differential equations make no sense to us, literally. Our perceptions are keyed to specific sensations. Roughness, weightiness, smoothness, sharpness, dullness. Foods are sweet or bitter or a combination of the two. Some things are warm and dry, others cold and wet. We cannot conceive of a reality not immersed in such responses. Not without faith in numbers. Trajectories and orbital mechanics. Energy and force. Momentum and inertia. Some of these are available to our senses. We all know what velocity feels like. But when someone tells us that there is more space in an ingot of steel than there is steel, we balk at the truthfulness of such a statement. We might readily agree, based on what we have learned in science. But it still seems beyond the reach of imagining. Because if there is more space than steel in an ingot of steel, what does that say about us? Are we ghosts? Clouds of atoms? Symphonies of molecules? Waves of light and radiant heat? All improbable, all incredible revelations. But the fact remains: a three-ton ingot of steel is mostly space. If an atom were the size of a 14-story building, the nucleus would be a grain of salt in the middle of the seventh floor.