Linda Sturms, age 11, of Scottsdale, Arizona, for her question:

How do we know that the earth isn't hollow?

A solid ball is heavier than a hollow one. Nobody has been down to prove with first hand information that the earth does not have a hollow inside. But we know a great deal about the weight of the world. If it were hollow, it would weigh a great deal less than it does. Of course, we cannot weigh the world on a pair of giant scales. Experts figure its weight partly from the pulling power of its gravity. This depends upon the amount of material packed inside the entire globe. If the earth were hollow, its gravity would be too weak to hold the golden moon in orbit. It might be too weak to hold us on the ground and spacecraft could zoom off into the universe with almost no fuel at all.

Experts have used a lot of indirect evidence to estimate the entire weight of our planet. They have also weighed and tested the minerals that form the surface crust. Far from being hollow, the center of the earth must be packed with heavier materials than we find in the rocky crust. What's more, for its size the earth weighs more than any other planet in the Solar System.