Linda Craig, age 11, of Rockford, Illinois, for her question:

Are we sure the world is not flat?

No matter how things look, we are quite sure that the world is round. If it were flat, there would be edges to it   we would travel so far and fall off. Instead of which, ships sail clear around the world and airplanes fly around it every day.

On the prairies, you can look in every direction and see the flat earth stretch out as far as your eye can see. This might lead you to suppose that the whole earth is indeed flat. But look again. Why can you see so far and no further? Why does the scenery end at the horizon? It is because you are standing on a big ball and the ground gently dips down on all sides. At the horizon, it curves out of sight. The surface goes on curving around to the other side of the globe.