Here's why:
* They breathe air: Killer whales, like all mammals, need to breathe air through lungs. They cannot breathe underwater like fish do.
* They give birth to live young: They do not lay eggs.
* They nurse their young: Killer whales produce milk to feed their babies.
* They are warm-blooded: They maintain a constant internal body temperature, unlike fish.
Killer whales belong to the order *Cetacea*, which also includes dolphins and porpoises. They are actually the largest member of the oceanic dolphin family.