Harvey Levine, age 9, of Winnipeg, Manitoba, Canada, for his question:

Is the sand on the Sahara desert very deep?

The great Sahara is 3,200 miles wide and big enough to cover most of Canada. It is, of course, the world's biggest desert. But the Sahara is not a vast stretch of sand from end to end. It has long, wide patches of bare rock and places covered with thin soil. It has mountains, some of them high enough to be crowned with snow. And here and there are fresh watery springs surrounded by rich soil and thriving farmlands. In fact, the sandy surface covers less than one tenth of the whole desert.

But the sandy region of the Sahara is as sandy as you imagine it to be, and perhaps more so. Most of it is in the southeastern part of the desert where scorching winds sweep low over the ground. And the dry grains of sand fly off on the breezes, traveling in duster clouds and piling up in high dunes around boulders and jutting rocks. The dunes may be low, rolling hills standing a few feet high on the desert's rocky floor. But sometimes strong winds pile giant dunes 500 feet high. Such a pile of Sahara sand needs another 104 feet to be as tall as the Canadian Imperial Bank of Commerce in Montreal. But unlike the building, the sand dune does not last. Tomorrow the wind may shift the sand to some other part of the desert, leaving a patch of bare, rocky floor.