D: Of course not!
Y: It just seems to be one of those things. The sun is round. The moon is round. The earth is round. What else would it be?
D: Why not a cube?
Y: A cube? No, that's silly.
D: Is it? Why?
Y: Because . . . because . . . (frustrated) . . . because really big things like to be round for some reason!
D: Exactly right. And that reason is gravity.
Y: Gravity?
D: Think of it this way. Suppose the earth were a cube. Then some points on its surface would be farther away from the center than others. The corners, for example, would be much farther out than the rest.
Y: I think I see where you're going. Since gravity works equally on everything, there would be a tendency to pull those uneven places down. The surface would even itself out.
D: Right. And the only geometrical object in which every point on the surface is the same distance from the center is. . .?
Y: A sphere! . . . But wait a minute. I've seen pictures of asteroids that have all sorts of crazy shapes. Why aren't they round?
D: Only because they aren't big enough for gravity to be a determining factor. If that asteroid were to suddenly become as big as a moon, it would indeed rearrange itself gradually into a sphere.
Y: Well, I never . . . !
