From The Kids' Book of Awesome Stuff (Brotman-Marshfield Curriculum, June 2004). Used with permission.
Take a deep breath. The air you just breathed in is different from the air you just breathed out. The air you breathed out is a gas that can suffocate you if too much of it gets in the air. That gas is carbon dioxide.
The oxygen you breathed in makes energy for you from the food you eat. But when your cells use the oxygen, they end up with something your body doesn't want: carbon dioxide. So your blood goes around to all your cells like a trash hauler, collecting the carbon dioxide and taking it to your lungs. You breathe it out.
All night, all day, every creature on earth breathes out carbon dioxide. Over and over. That's a lot of suffocating gas! A little carbon dioxide in the air is normal and safe. But if the air around you filled up completely with carbon dioxide (or even just halfway), you would suffocate.
So . . . with all the breathing out that goes on in the world, what keeps you from smothering under a thick cloud of carbon dioxide? What saves you?
You are saved by something green.
All over the planet, green plants on the land and in the ocean pull carbon dioxide gas out of the air by breathing it in themselves! The very same gas that can kill you is exactly what a green plant must have to live.
Plants don't breathe with lungs like yours, of course. Instead, they breathe in the carbon dioxide through millions of invisible holes on the underside of their leaves and on their stems. The tiny openings can close and open.
Every time a leaf breathes in carbon dioxide, it also breathes out an equal amount of pure oxygen. The leaf is getting rid of oxygen it doesn't want.
The same oxygen that leaves breathe out is exactly what you need to breathe in. You can't live without it. You breathe in what leaves breathe out. So do slugs. So do crows, crocodiles, and crickets . . .
Oxygen from plants travels on wind currents from far-away continents and oceans. It reaches you even when the trees around you have lost their leaves, or if you live in a place with no leaves at all. Right this minute, you may be breathing oxygen from a rain forest in Costa Rica, or a field of red poppies in Turkey.