Scientists group animals based on key traits or characters that define them. Do you know what some of those traits are for mammals? I bet you do! Think about: warm blooded, make milk for their babies, and have fur or hair... you can look up the rest. Guess what? Whales have those traits! Now think about fish - do they need to breathe air? Nope - they have gills that can filter oxygen from the water. But whales have true lungs like yours - they have to inhale a big gulp of air and hold their breath when they go under water. Now of course their lungs are bigger and shaped slightly different than a person's, but they are still lungs. Here's another question for you to think about (and I bet you'll figure it out): is a bat a bird? Why or why not?

So mammals are warm-blooded animals that have hair and usually give live birth, rather than lay eggs. Fish on the other hand, are cold-blooded and lay eggs. So if an animal swims in the water, but still gives live birth, it is probably a mammal and not a fish. Whales are interesting because they were fish a long time ago, then became a wolf-like land creature, and then evolved back into the water. So every sea mammal was once a land mammal, but fish have never lived on land.

Wales give live birth, make milk, have a three-bone inner ear, and breathe air. Fish do none of these things, but mammals do all of them. Therefore, whales are mammals.

Whales and fish are marine creatures but whales are categorized as mammals because of certain characteristics. Mammals are usually endothermic, breathe with lunge, give birth to live young, and have hair. Whales are endothermic meaning they are warm-blooded, and their internal temperature is maintained and doesn't change based on their environment. They have to come to the surface and breathe through their blowholes, whereas fish use gills to breathe and just remove oxygen from their bodies. Whales give birth to live young whereas fish lay eggs. Lastly, even though they aren't covered in hair, they do have bristly hair around some parts of their bodies. ​Even though whales look like "big fish", they are completely difference than fish and are categorized as mammals.

Whales are mammals even though they look like fish. Whales, like other mammals are warm blooded, meaning they keep their bodies warm. Whales keep their body temperatures higher than the water they swim in. Whales also give live births. That means that their whales' babies are miniature versions of the adult; they don't lay eggs. The whales also feed their babies milk, another characteristic of mammals. It is true that most mammals usually have fur but whales don't. But whales have most of the characteristics of mammals, so they belong in that group.