Robert Jack, age 10, of Huntsville, Alabama, for his question:

Why does hot air rise?

The air is invisible so we cannot watch what happens to it. And some of these things are hard to believe. For instance, it is hard to imagine that air is made from zillions of tiny molecules. What's more, they dash around like zillions of mini baseballs    all whirring in different directions. And this is not all. If things get warmer, they zoom around faster.

When the temperature rises, air molecules move faster and spread farther apart. As the air spreads out and expands, it gets thinner. It also gets lighter  ¬and zooms upward, somewhat like a cork floating to the top of a pond. Outdoors, a pocket of warm air may be surrounded by cooler air on all sides. If it tries to spread sideways, its molecules lose their pep. So the natural way for warm air to go is upward.