Susan Blankenship, age 12, of Asheville, North Carolina, for her question:

Why are many creek pebbles round?

There is an old saying that dripping water can wear away the hardest stone. This is true. It may take a long time, but if the water keeps on drip drip dripping, it can wear down a granite boulder or dig a hollow in hard, man made cement. Running water can do the same thing, and do it better. The reason is that water is a natural solvent, always eager to dissolve invisible fragments from rocks and other minerals. Flowing water carries these dissolved fragments away, stealing atoms and molecules from the rocks as it goes.

A lazy creek is not in a hurry. But it has been flowing gently over the same old rocks for years and years. Running water pushes harder at the jagged edges in its path. It tends to dissolve them away first. In time, even the laziest creek washes the bumps and corners off all the stones on its floor. In swifter streams, rocks and sand tumble along. Their rubbing action on stones also tends to make them smooth and round.