Here's why:
* Breathe air: Whales have lungs and must surface to breathe air, unlike fish who breathe underwater using gills.
* Give birth to live young: Whales give birth to live offspring, which they nurse with milk. Fish lay eggs.
* Warm-blooded: Whales are warm-blooded, meaning they can regulate their own body temperature. Fish are cold-blooded, meaning their body temperature changes with the surrounding environment.
* Have hair: Whales have hair, though it is often sparse and difficult to see. Fish do not have hair.
While whales live in the ocean and have some fish-like features, they share many characteristics with other mammals, making them a part of the mammalian class.