Choose the odd one out: Shark, Whale, Dolphin, Porpoise.

The whale, the dolphin and the porpoise are mammals: they come up to breathe air through a blowhole and suckle their young. The shark is a fish, drawing oxygen through gills and never giving milk. All four spend their whole lives in the sea and are shaped much alike, which is precisely why the shark passes for one of them, but the group is held together by being mammals.