Whales aren't fish, but mammals; they are warm-blooded, breathe air and nurse their babies with milk. Many species of whales grow very large; the blue whale can grow to over 90 feet long. Whales evolved from creatures that lived on land and returned to the sea millions of years ago.
A whale's nostrils sit on the top of its head. This placement allows a whale to breathe by simply barely breaking the surface of the water, rather than having to lift its whole head out of the water.
Whales are carnivorous, but some whales such as sperm and killer whales have teeth. Other whales have plates of horny material in their mouths called baleen. They use baleen to strain their food out of the water. There are two types of baleen whales: rorqual whales and non-rorqual whales. Rorqual whales have grooves on their throats that expand when they gulp large amounts of water and fish.
Because it can be difficult to see in deep water, toothed whales have developed echolocation. They send out sounds that strike an object and bounce back to them. Echolocation is so accurate that it lets the whale know whether the object is an obstacle like a seamount or potential prey.