Rowena MacnJfullen, age 15, of Coblcskill, N.Y., for her question:

Why doesn't a lake dry up?

rornnps you know a smooth lake which stays at the same level, more or less, year after year. Certainly some of its water evaporates into the air and you wonder why it does not dry up and disappear. Something must be at work to keep it filled. And this is how a lake works like a big bath tub. It has a faucet and a drain. It has a supply of water and a way to got rid of water. The supply of water may come from one or morn springs on the floor of the lake. Or it may come from a tiny crock, so small thnt you have to tour the rim of the lake to find it. You may also have to hunt for the drain, the creek which takes water away from the lake.

Any lake which endures must be fad by springs or creaks. It must have an outlet, or evaporation would make it salty.