Linda Pinkowski, age 11, of Gurnee, Illinois, for her question:

How do we get oxygen when the winter trees are bare?

The filmy atmosphere wraps around the entire globe, merging with land and sea and reaching up hundreds of miles above our heads. Near the surface, everywhere its mixture of gases is more or less the same from pole to pole. Summer and winter it contains about 78 per cent nitrogen and 21 per cent oxygen, plus traces of a few other gases and various helpings of pollution particles. We know that oxygen is contributed by the green plant world and that many trees lose their leaves during the winter season.

This makes no difference to the amount of our oxygen, for the breezes distribute the supply around the globe. What's more, a large portion of the oxygen in the air comes from teeming little green plants in the sea. And the salty seas cover the largest part of our planet. When our trees lose their leaves, forests of evergreens continue to pour out oxygen and countless tons of oxygen are added by zillions of tiny plants that swarm in the oceans from pole to pole.