Rachel Hargrove, age 9, of Huntsville, Alabama, for her question:

How much water evaporates into the air?

We cannot measure this in exact teaspoonfuls. But scientists can estimate how much liquid water evaporates from the earth and becomes gaseous vapor in the air. An estimate is figured by taking an enormous number of small samples. From this we can say that about 95,000 cubic miles of water evaporates from the earth every year. Most of it is picked up when the sun shines on the surface of the enormous seas. All this vapor stays up in the air and the clouds for awhile. Then it turns into rain or snow and falls back down again.

It is hard to imagine all this water that cycles up and down between the air and the earth. One cubic mile of it is enough to fill a square tank that is one mile long„ one mile wide and one mile high. Every day, enough water to fill about 260 of these tanks evaporates into the air. And somewhere else, the same amount comes falling back as rain or snow. This non stop elevator around the world is called the water cycle.