Water is a chemical substance that is composed of hydrogen and oxygen and is vital for all known forms of life.  In typical usage, water refers only to its liquid form or state , but the substance also has a solid state, ice , and a gaseous state, water vapor , or steam . Water covers 71% of the Earth 's surface.  On Earth, it is found mostly in oceans and other large water bodies, with % of water below ground in aquifers and % in the air as vapor , clouds, and precipitation .   Oceans hold 97% of surface water, glaciers , and polar ice caps %, and other land surface water such as rivers, lakes, and ponds %. Additionally, a minute amount of the Earth's water is contained within biological bodies and manufactured products.
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Both of the sets of natural numbers defined above are denumerable. They are also exactly the same size. It's not difficult to prove this; their elements can be paired off one-to-one, with no elements being left out of either set. In infinite sets, the existence of a one-to-one correspondence is the litmus test for determining cardinality , or size. The set of integers and the set of rational numbers has the same cardinality as N . However, the sets of real numbers, imaginary numbers, and complex numbers have cardinality larger than that of N .