Tammy Stanfill, age 10, of Enid, Oklahoma, for her question:

Why do balloons float up through the air?     

This operation is related to a cork bobbing on the waves. We know it floats because cork is made of lighter material than water. Actually it is light because its empty pockets are filled with air. And the air itself floats above the earth because it is lighter than the solid land and the watery seas. This peculiar quality is called buoyancy. It also applies to what goes on in the air, because air is a mixture of gases and some gases are heavier, or denser than others.     

Carbon dioxide is 1 1/2 times denser than ordinary air. A balloon filled with this gas cannot float in still air because it is too heavy. The light weight gases are helium and hydrogen. They are buoyant enough to float up through the air. Hydrogen makes the best lifting gas, but we don't use it because it tends to burst into flames. Helium is almost as buoyant and we can trust it not to burn.