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Air is a mixture of gases that circle Earth, kept in place by gravity. Air makes up Earth's atmosphere. The air we breathe is 78 percent nitrogen gas, 21 percent oxygen, 0.9 percent argon, and 0.03 percent carbon dioxide, along with water vapor-floating molecules of water. Also present are traces of other gases and tiny bits of dust, pollen grains from plants, and other solid particles. As our atmosphere extends higher and higher above Earth, toward outer space, air becomes thinner and the combination of gases in the air changes.