Kerry Bince, age 10, of Calgary, Alberta, Canada, for his question:

Why doesn't the air go up into outer space?

The air around the earth is made of molecules of gas and molecules are particles of matter, even when they are too small to be seen. The force of the earth's gravity grips the air, just as it hugs us to the surface of the globe and pulls down a falling stone. Gravity keeps our air from escaping and going off into outer space.

But the earth's gravity loses its strength as it reaches farther and farther out into space. Hundreds of miles up from the ground, it begins to lose its grip on the air. The air gets thinner and thinner because the pull of gravity is too weak to hold it. Maybe a few molecules at the very top do escape and go off into outer space. But the rest of the air must stay close to home because gravity hugs it down and will not let it escape.