@shangyuangua39
Why can fish breathe the oxygen in H2O, but not air? Do they separate oxygen from H2O? If so, what happens to the other hydrogen?
The oxygen that fish breathe is not the “O” in H2O.
Fish don't breathe oxygen in the water molecule; that molecule remains intact. What exists in the water is atmospheric O2 dissolved in it. That's what they breathe.
Now, why can't they also breathe oxygen from the air if it's the same oxygen? This question is even more interesting. Fish breathe through gills. The structure of the gills behaves quite similarly to human hair.
When a person is immersed in water, the strands of hair float and separate, and the surface of each strand has maximum contact with the fluid, in this case the fluid is water.
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