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Air takes up space because it is made of particles. Gravity pulls on these particles and keeps them near Earth. This is why we have an atmosphere and why we are able to breathe. Air in our atmosphere is made of mostly nitrogen and oxygen. Since it has mass, or stuff, it also has volume. The density, or thickness, of air changes the farther away you get from Earth. The higher you go above Earth's surface the less air there is until there is none left! You can find out more about air at How Things Fly!