Peter MacInnis, age 13, of Huntington beach, Calif., for his question:

WHAT IS OSMOSIS?

Osmosis is a movement of liquid from one solution into another through a membrane that separates them. Osmosis, for example, helps to regulate the flow of water between body fluids and cells. Also, plants absorb most of their water by means of osmosis.

Recently chemists have been using a process called reverse osmosis to purify water. Ordinarily, fresh water will flow into seawater when the two are separated by a semipermeable membrane. However, if pressure is applied to the seawater, the movement of water reverses itself and fresh water is produced from seawater.

Plant roots take in water as a result of the osmotic process. Osmosis also helps to move water in a plant and to maintain a plant's shape and stiffness.