How do fish breathe underwater?

Fish need oxygen just like us. But they do not breathe air — they take oxygen out of the water.

Water has a small amount of oxygen mixed into it. A fish opens its mouth, takes in water, and pushes it out through slits on the sides of its head called gills.

Gills are full of thin red folds packed with tiny blood vessels. As water flows over them, oxygen moves from the water into the blood. Then the blood carries it around the body.

That is why a fish dies out of water. The gills stick together in air and cannot work. And it is why fish in a dirty, warm pond gasp at the surface — warm, dirty water holds very little oxygen.