Fish are animals that live in water and breathe using gills. Water goes in through the mouth and out through the gills, which take oxygen from the water. Most fish swim by moving their tail (also called the caudal fin) left and right.
There are many kinds of fish; some have bones but others, like sharks and rays have no bones, only cartilage.
The biggest fish in the world is the Whale Shark; it is a shark but not a whale. The whale shark is up to 46 feet (14 m) long and weighs up to 15 tons.