Linda Popadic, age 10, of Stratford, Conn., for her question:

Where does the air we breathe come from?

Air is a mixture of gases and the gas important to us is oxygen. Without oxygen, none of us could survive for more than a few minutes. Oxygen is also necessary to plant and animal life. Countless tons of it are being used up from the air day and night.

Naturally this makes us wonder whether we shall ever run out of the precious gas. The answer is, No not so long as the plants survive and stay green. For green plants take the carbon dioxide from the air. This is the waste gas we breathe out. It is made from carbon and oxygen. Green plants take this gas from the air and use the carbon in it to make plant sugar. In this process, the oxygen is a waste material. The plants return it to the air. Some of this oxygan the plants use themselves, but there is enough left over to provide us with all the good fresh air we need.