A man walked into a movie theater and picked a prime seat. As he was starting to munch on his popcorn, he noticed what looked like a whale sitting a few seats over. Flabbergasted, the man asked, "Are you a whale?"

"Yes, of course," said the whale. "Umm…what are you doing at the movies?" asked the man. "Well," said the whale. "I really liked the book." "Ha! You're a funny fish!" exclaimed the man.

OK, so that's a bad joke, but do you know what's wrong with it? If you said whales don't go to the movies, you're on the wrong track. If you said whales aren't fish, then you were paying close attention!

Even though they live in the ocean, whales are mammals. That means they've got something in common with you and me, because human beings are mammals, too. But what exactly does it mean to be a mammal?

Mammals are the animals that make up the scientific class Mammalia. Throughout the entire Earth, there are over 5,000 different species of mammals. That sounds like a lot, doesn't it? Compared to the over 21,000 kinds of fish and over 800,000 kinds of insects in the world, though, mammals make up a fairly small group of animals.

Mammals range from the very tiny Kitti's hog-nosed bat that weighs less than an ounce to the extremely huge blue whale that can weigh as much as 150 tons. If you put a hog-nosed bat, a human being, and a whale in the same room, you'd need a very large room, but you'd also notice how different these creatures are.

Despite their differences, though, all mammals share some common characteristics. For example, all mammals are vertebrates. That means that they have a backbone. They also have three middle ear bones.

All mammals have hair of some sort, as well as mammary glands. In female mammals, the mammary glands produce milk that they use to feed their young. Mammals are also warm-blooded, which means they regulate their own body temperature via a region in the brain called the neocortex.

Mammals come in all sorts of shapes and sizes. They can also live just about anywhere on Earth. From whales and dolphins in the sea to elephants and primates on land, mammals have found homes in a variety of places, including in trees and in the air.

There is one other common characteristic shared by nearly all mammals with just a few exceptions: nearly all mammals give birth to live young. The exceptions to this rule are monotremes, such as the echidna and platypus, which lay eggs instead of giving birth to live young.

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