Barbara Davey, age 10, of St. Catharines, Ontario, Canada, for her question:

How can clouds stay up in the air?

Most of the things in this world are very tiny. Compared with these minuscule ob¬jects, your body is a monstrous giant. The air is made of zillions of tiny gas particles. But these particles are so small and the airy mixture is so thin that we tend to forget that it is there. True, we notice it when the forceful wind blows it in our faces. But every time we move even a finger we push around zillions of gas particles with no trouble at all. Certainly the filmy air is too thin to hold our bulky bodies above the ground. But remember all those tiny objects. Some of them are misty droplets of water.

A cloud is made of tiny misty droplets of water. The zillions of these cloud droplets are separated and far apart. Each one is small enough to stay aloft in the air at least for a time. Actually it is being pulled down by the earth's gravity but the air slows down its fall. With other droplets, it can stay aloft for days while the misty cloud is blown along like a flying carpet by the breezes. But slowly, very slowly the cloud is sinking. At last it may settle on a slope and become a fog. But as a rule, the cloudy mist changes into rain before this happens. And raindrops tumble down because they are too big and heavy to float aloft in the filmy air.