Mammals are animals that
1) require air to breath,
2) are warm blooded, so depend on a constant supply of food to stay warm,
3) have fur or hair,
4) have females that produce milk, and
5) give birth to live young (they don't lay eggs like a chicken).
Whales fit into this category!

You don't typically think of whales as being hairy, but it turns out that the baby whales of some species possess hair when they are young and then loose it. Whales have blubber to keep them warm so may have lost hair over the course of evolution. But, we still consider them mammals.

Mammals are animals that give their children milk as food. They also have a backbone, a single bone for their lower jaw, four-chambered hearts, a diaphragm (a sheet of muscle and tendon that separates the body cavity), and complex brain function. Even though whales look fish-shaped, they have all of these things. They also feed their babies with milk! Since whales meet all the criteria of being mammals, they're classified as mammals.

Whales are hoofed mammals, related to hippos, but have evolved to live in the ocean, losing most of their land-going traits (like hooves). Like all mammals, they make milk, and have three bones in their middle ears. They have lost hair, and also give live birth.

The requirements of being a mammal are:
#1 Has mammary glands that produce milk to feed its young (which is why they're called "mammals"!)
#2 Has a little or a lot of fur (whales are born with a mustache!)

There are some other requirements as well, like having a diaphragm, but those two are the most important. Since whales can feed their young with the milk they produce and are born with hair, they are considered mammals. There are about 4,260 named mammal species in the world.

All mammals share two characteristics. One, they must feed their young with milk produced from the mother's mammary glands. Two, they must have hair, even just a little. Whales produce milk to feed their babies. They also have some hair when they are young. However, most species lose hair when they become adults. Fun fact: baby humpback whales drink 150 gallons of milk a day.