Claudette Mathis, age 11, of Kirkland, Wash., for her question:

How does oxygen get into the air?

A little oxygen may seep out of geysers and underground cracks in the earth. A little bitsy free oxygen may seep out from the oceans and rivers. But almost all the oxygen we breathe is a gift from the plant world. All the green leaves and stems pour forth oxygen throughout the daylight hours. Every day, billions of tons pour forth from the green forests and jungles. The restless winds carry it all around the world.

A green plant uses carbon dioxide in the air, water from the soil and sunlight to make simple sugar. This recipe is for the food it uses to meet all its needs. Carbon dioxide the plant uses is the waste gas we breathe out and in the process it returns oxygen to the air. This give and take provides the plant with its sugar recipe and gives us oxygen.