When we breath, we draw air into our lungs and take from it the oxygen we need in order to remain alive. Like other animals, fish need oxygen in order to live. But a fish gets its supply of oxygen not from the air, but from the water into its mouth. Then it closes its mouth and squeezes the water over its gills. These are feathery pieces of tissue on each side of its throat, and they are full of tiny blood vessels. Just as our lungs take oxygen out of the air, so the gills take oxygen out of the water. And just as with us, so the waste carbon dioxide is passed back through the gills and the used water then flows out through the fish's gill slits. There are some exceptional fish which breath through their skins and has lungs as well as gills.