How do fish breathe underwater?
For grown-ups: the science
Fish use gills to move oxygen from water into their blood.
Water contains a relatively small amount of oxygen gas dissolved in it. As a fish pumps water across its gills, oxygen diffuses across thin, blood-rich gill filaments into the blood, while carbon dioxide diffuses from the blood into the water. This is gas exchange, similar in principle to what happens between air and blood in human lungs.
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Transcript
A silver fish swishes through a pond. How can it get the oxygen it needs down there?
Pond water holds a little oxygen gas, even though we cannot see it. A fish needs that oxygen, just as we do.
A fish pumps water in through its mouth and over its feathery gills. The gills have thin parts with blood flowing through them.
Oxygen moves from the water into the fish's blood through the thin gills. Carbon dioxide moves out of its blood and into the water.
So that is why fish can live underwater: their gills move dissolved oxygen into their blood. Carbon dioxide moves back into the water.