Talking about mammals may be equated to reviewing a burger joint's menu. Take the Original -- the most basic hamburger the restaurant serves. The Original, just a patty on a bun, represents your standard definition of mammals: warm-blooded vertebrates that have hair and milk-producing glands.

So what happens when you start dressing the Original with onions, cheese or something a little wilder, like guacamole? In the mammalian world, these condiments represent all the different available varieties. Prefer your mammal with a pouch, and you're treated to the marsupial; want one to arrive in its original egg packaging, and you've got the monotreme. The variety of species within the Mammalia class is staggering.

Some animals might make you scratch your head as to why they were dumped into the mammalian class. After all, when was the last time you saw a hairy dolphin or whale? However slight or unobvious to the naked eye, every animal labeled a mammal meets the qualifications. And for your information, dolphins have a few bristles near their mouths, especially when they're young.

Behaviors once thought exclusive to humans can be seen among other mammalian species. Using tools, showing emotion, communicating and demonstrating parenting skills are traits seen in many mammals. From orangutans that dutifully care for their young to elephants that exhibit signs of grief and anger, there seems to be no limit to what mammals can do.

So can you order your mammal with whiskers, hold the mammary glands? No way. Even though there are some ways to tweak the recipe (flyer or swimmer, insectivore or carnivore, hooves or paws), certain ingredients can never be changed.