They are the only mammals, other than manatees, that live their entire lives in the water.
Some whales are huge.
The blue whale, for example, grows to 100 feet in length and over 200 tons in weight. It is the largest animal that has ever lived on earth. Some whales, such as the dolphins, grow only three feet long. Like other mammals, such as cats, dogs, deer, mice and humans, whales are warm-blooded. Whales breathe air and nurse their young.