Kathy Hanson, age Il, of Middleton, Wisc., for her question:

How did the Sahara get all that sand?

Sand is made of gritty grains of silica, the hardest of the common minerals in the earth's crust. softer minerals tend to blow away and dissolve in running water. The Sahara is a vast desert where the sun beats down and the dry ground is parched for weeks and even months.

The arid heat causes the earthy minerals to crumble into dry fragments. The desert winds blow away the fine fragments of soft dirt, and other soft minerals are floated away in the streams that gush over the ground after the rare showers. With years of sifting, the desert loses most of its soft, crumbly minerals, but the sturdy grains of hard sand remain.