This is a tricky question. The sand in the
comes from broken rock just like the soil in your
yard. The difference is that in the desert there
are few plants and almost no water. Rocks break
up into many different size pieces. When there is
water around, those pieces are carried away by the
water. If there are plants, their roots can act
like a screen that will hold the pieces. In the
desert, there are few plants to hold soil in place
and little water. Instead, wind is the main way
that the rock pieces are moved around. The wind
cannot move all the pieces though, just the small
lighter ones, so sand is left behind. Over a long
time the deserts become mostly sand.
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