Vikki Chandler, age 10, of Newport News, Va., for her question:

WHY IS THERE NO AIR ON THE MOON?

This has something to do with gravity, the pulling force that hugs us to the surface of the earth. The earth happens to be a sizable planet, very heavy for its size. The strength of its gravity depends on its size and weight. Actually, the earth's gravity is strong enough to reach out and hold down our airy atmosphere, which reaches hundreds of miles above our heads.

The moon is only about one quarter as big as the earth, and it is made of much lighter materials.  This is why gravity on the moon is six times weaker than it is on the surface of the earth. Everything on the lunar surface weighssix times less than it does on the earth. And this weak gravity is not strong enough to hold on to an airy atmosphere.    The gaseous molecules would escape and get lost in space.