Some sea animals breathe underwater, and others
just hold their breath a really long time. Whales
and dolphins and other mammals (and birds too!)
are in this category. They just stock up on air
before going underwater. Fish and amphibians
(like tadpoles) breathe with gills that take air
out of the water. When water flows and churns,
air mixes into it, and fish can breathe this air
out of the water. If you have pet fish, there's
always water running somewhere in the tank because
if you let it get perfectly still, there wouldn't
be enough air mixed in and the fish would struggle
to breathe. Gills are very interesting organs, and
need to stay wet to work, so fish can't actually
breathe out of water.
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