Here's why:
* Phylum Chordata: This phylum includes all animals with a notochord (a flexible rod that supports the body) at some point in their development. Whales have a backbone, which is a more developed version of the notochord.
* Class Mammalia: Whales are mammals because they share key characteristics with other mammals, including:
* Warm-blooded: They maintain a constant body temperature.
* Give birth to live young: They don't lay eggs.
* Nurse their young with milk: They have mammary glands to produce milk for their offspring.
* Have hair (at some point in their life): While most whales don't appear hairy, they have some hair follicles, especially when they are young.
It's important to note that whales are further divided into two suborders: Odontocetes (toothed whales) and Mysticetes (baleen whales).