Marsha Ackerman, age 12, of Philadelphia, Penna., for her question:

What is the water cycle?

A cycle is something which goes around and around with no beginning and no end. The water cycle goes up, floats over the sky, comes down, runs along the ground and goes up again. And every day it moves millions of tons of water. All of this happens so smoothly that we hardly notice it. It begins with the smiling sun. Every day, millions of tons of water are evaporated up from the sea, from the rivers and from puddles.

The water is now vapor. Once aloft, it turns into misty droplets and forms the clouds. The clouds are blown along by the wind, but sooner or later their moisture turns into raindrops and comes tumbling down again. The rain water feeds the plants and moistens the soil. Some sinks into the ground and some runs away to form rivers and sooner or later the rivers join the sea. Meanwhile more water is evaporating to start its trip around the water cycle.