Susan McQuiston, age 10, of Huntsville, Alabama, for her question:

What stops our air from wearing out?

All the plants and animals take oxygen from the air, just as we do. They also return waste carbon dioxide to the air, just as we do. This takes tons of oxygen from the air every day. Yet there is always enough to go around. This is because the plants change our waste carbon dioxide back again into oxygen. This fresh oxygen from all the greenery mixes with the air and the winds waft it around the world. Nature likes her children to share things. The plants take our waste carbon dioxide and give back fresh oxygen. We take their waste oxygen and give them back our waste carbon dioxide.

The plants also use up oxygen as we do. But their own waste carbon dioxide is not enough for them. They need tons more to make their plant food. This plant food recipe is called photosynthesis. It uses sunlight, water and carbon dioxide. The plants need our extra carbon dioxide to make their food. And the photosynthesis recipe gives off oxygen as a waste gas. The plant world gives off enough oxygen for itself and enough for us also. We give the plants our waste carbon dioxide and they give us their waste oxygen. We take their oxygen and they take our carbon dioxide. This give and take between us and the plant world stops the air from wearing out.