Water is made of hydrogen and oxygen gas. The reason that we can't breathe water is is because the oxygen is bonded together with 2 atoms of hydrogen and the oxygen is then useless to our lungs in this form. Also, humans can't breathe underwater because our lungs don't have enough surface area to absorb enough oxygen from water as there is not that much in it. The lining of our lungs, also is adapted to handle air rather than water. However, scientists have done experiments where they make humans breathe other liquids like fluorocarbons. Fluorocarbons can dissolve enough oxygen so that we can breathe.
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