Jennifer Welton, age 11, of Eugene, Oregon, for her question:

How deep is the Sahara's sand?

Africa's great Sahara Desert is almost as big as the entire United States. Naturally in such a vast area there are changes of scenery. In some places there are miles and miles of bare, gravelly rocks. Also there are long ranges of bare, craggy mountains. Here and there we find an oasis, where underground water feeds a patch of greenery. Also there are miles and miles of shifting, windblown sand dunes. Actually, the sands cover only about one tenth of the great Sahara. And these sands rest on solid bedrock below. As the wind blows, it piles the sands in hilly dunes that change their shapes with the breezes. Often the sand in the dunes is 30 feet deep and often the winds strip the underlying bedrock bare. Sometimes the sands pile up 500 feet deep and build a hilly range that is many miles long. Chances are, the changing winds eventually will shift this great pile of sand somewhere else.