Mike Nelson, age 13, of Washington, Illinois, for his question:

What causes warm spots in a lake?

If a lake has been there a long time, you may be sure that its water has not. It may look serene and still, but the water is flowing gently in and out. A lake is somewhat like an enormous outdoor bathtub with inlets and outlets. Sometimes the water enters and leaves by streams. Sometimes it is fed by springs of water welling up from below the ground.

The surface of the lake is warmed by the sun and its temperature changes with the seasons. The springs and streams that feed it may be warmer or cooler than the rest of the water. When you go swimming, you notice these hidden sources as warm or cool spots. In summer, spring water tends to be cooler than the lake and secret streams may bring in spurts of warmer water. In mountainous or volcanic regions, the warm spots in a lake usually spurt up from hot springs. No two lakes are exactly alike and almost all of them are fed by sources of warmer or cooler water.