Patricia Klerowski, age ll, of SomervilIe, Mass., for her question:

What is an oasis made of:

It is a small area that appears suddenly in the midst of stretches of arid desert. There is a waterhole or a well or perhaps several of them. There are palms and a small area of the sandy wasteland is carpeted with greenery. This is the surface of the oasis. Below the thin layer of moist topsoil are the bedrocks that make it possible.

The water table of a desert sits on deep layers of bedrock, and the sparse rainfall quickly sinks down far below the surface. But the bedrocks that support the world's ground water are uneven. even in the desert there are places where a dense ledge or basin rises up just below the surtace. Here the rainwater becomes trapped. It moistens the topsoil and creates an oasis of greenery.