Kids ask a lot of questions.

mental_floss has answers. This week we launched WHY?, our new series for kids and parents. We'll tackle all types of questions children have about how the world works by providing science-based, kid-friendly content. Our answers are written with early readers (ages 4 to 7) in mind, but we think they're interesting—and educational—for everyone.

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Air surrounds the surface of the Earth. Air is a mix of gases: 78 percent nitrogen, 21 percent oxygen, 1 percent argon, a little bit of carbon dioxide, and tiny amounts of 13 other gases. Plus, air contains water vapor, a mix of hydrogen and oxygen. Air is the part of the atmosphere closest to the Earth. Like a big blanket surrounding Earth, the atmosphere keeps us from getting too hot from the sun or too cold from space—and allows us to breathe. 

Land animals can’t exist without the oxygen in the air. More than 2.3 billion years ago the atmosphere was mostly nitrogen. But then the number of tiny microbes called cyanobacteria (SIGH-an-oh-back-TEER-e-uh) grew. Cyanobacteria use sunlight to change carbon dioxide into a kind of food—a process called photosynthesis. The waste product of photosynthesis is oxygen. So much oxygen was created that it began to fill up the atmosphere. After time, more animals evolved that were aerobic (air-OH-bick), meaning they need oxygen to survive—like us. 

Because plants photosynthesize! Remember, oxygen is a waste product of photosynthesis, and an important part of air. Without oxygen, we can’t breathe. Thanks for making oxygen, plants! But we do something for plants too. We breathe out carbon dioxide, which plants convert into simple food. So plants and animals work together to keep all creatures alive. How cool is that? 

For fun further reading about photosynthesis, visit this gardening guide for kids from the BBC.