David Powell,age 11, of Pittsburg, PA  for his question:

What is the biosphere?

The bio part of this word means life or living things. A sphere, of course, is a round ball. The biosphere is the round outer skin of the earth where living things can exist. It includes the seas, teeming with fish and water plants. It includes the soil, teeming with worms and all sorts of insects. It includes the air with the treetops and the flying birds. It also includes the surface of the dry continents and all the plants and animals that live there.

We tend to think that the biosphere includes the whole earth actually it is a very thin layer of earth, air and water. The distance from your feet to the center of the earth is about 4,000 miles. On land, the biosphere is less than a mile deep and perhaps three miles deep in the oceans. The air reaches hundreds of miles above our heads but the biosphere does not reach as high as Mount Everest, which is five and a half miles above sea level. Actually the biosphere, the layer in which conditions are right to support living things, is less than ten miles thick.