Chris Gerber, age 10, of Peoria, Illinois, for his question:

Do bridges and tall buildings make the earth heavier?

Many of Andy's young pen pals wonder about this problem. The answer is that none of these man made structures add an ounce to the weight of the world. If you think about it, you can understand why this must be true. The weight of the world is estimated to be about 6,570 quintillion tons. You can write this stupendous figure as 6,570 with a tail of 18 zeros. And this stupendous weight includes our solid globe, the atmosphere above it, the oceans, every single stone and other object on the planet's surface.

Metals and building materials are taken from the earth's crust and all of them have already been included in the overall weight of the earth. By arranging them in buildings, we do not add to the earth's total weight. However, the earth is gaining weight every day. Ashes from burned out meteorites sift down through the air and solid meteorites land on the ground. This debris comes from space and every year it adds hundreds of tons to the weight of the world.