"The colorless, odorless, tasteless mixture of gases that surrounds the Earth. Air consists of about 78 percent nitrogen and 21 percent oxygen, with the remaining part made up mainly of argon, carbon dioxide, neon, helium, methane, and krypton in decreasing order of volume. Air also contains varying amounts of water vapor, particulate matter such as dust and soot, and chemical pollutants."1
All animals need to breathe oxygen, a gas which is found in the air and water. All animals take oxygen into their bodies, in a variety of ways (Fish have gills, which filter oxygen from the water as it flows through them, many insects take in oxygen through tiny holes in their bodies, called spiracles. Humans and many other animals have lungs, which extract oxygen form the air .2
All plants take in carbon dioxide, a gas found in the air, through tiny holes in their leaves called stomata. Stomata let carbon dioxide in, and water and oxygen out .2
1.The American Heritage® Science Dictionary Copyright © 2005 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.
2. Claybourne, Doherty, Treays. 2000.The Usborne Encyclopedia of Planet Earth. Various pages on Air (48,95,99)