Laura Leming, age 9, of Sarasota, Fla., for her question:

What is a lighter than air craft?

A cork floats on the top of the waves. This is because cork, so full of holes, is lighter than the water around it. A stone sinks because it is heavier than water. Almost everything is heavier than the light and filmy air. This is why, when you toss a ball or a stick, it rides a little way, but always falls down through the air to the solid ground.

But a few things in the world are lighter than the air, and they will float on and on above the ground like corks float on the water. The air is a mixture of gases, and some of the gases weigh less than others. Pure hydrogen gas and pure helium gas are both lighter than the air. If we fill a balloon with one of these lighter than air gases, we have a lighter than air craft. Hot air is lighter than cold air, and a balloon filled with warm air will float above the ground until the air inside becomes as cool and as heavy as the air around it.