David Schulman, age 15, of Louisville, Ky., for his question:

What holds the world together?

Our planet is held together by the force of gravity. Gravity also keeps the earth's blanket of atmosphere shrouded around it. And it is gravity which dictates the shape of a planet. This is the reason why our earth and the other planets are more or less spherical and not square, oblong or any other shape.

The force of gravity, we arc told, operates to pull everything towards the center of the earth. Rocks and other massive material tend to sink towards the earth's center, with the heaviest materials centered in the earth’s core. Lighter and lighter layers of rock rest, or float, upon this heavy core.

This powerful force operates from the center of gravity outwards in all directions. It holds everything in shape. Even if, by some wild stretch of the imagination, the earth were shaped into a ‑square, the force of gravity would in time pull it back into a sphere.