Paul Martin, age 9, of New Hartford, New York, for his question:

What makes a flame go up?

There is a reason why this is the proper way for a flickering flame to go. But the reason is not easy to understand because our eyes cannot see why it happens. We have to depend on what the experts tell us about how gases behave. And most gases, of course, are invisible. But, even though we cannot keep an eye on them, they must obey certain very strict rules. For example, when gases get hot they spread out and expand. This makes them lighter    which should give you a hint.

Cork floats because it is lighter than water. A balloon filled with helium soars aloft because helium is lighter than the ordinary air. Hot air rises when it expands enough to become lighter than the air around it. Fires and candles warm pockets of air and set some of the gases a blaze. The proper way for the flames to go is up, because their gases are lighter than the air around them.