In this lesson you will learn how to classify animals.

Animals may be classified in many ways. They may be classified by where they live, what they eat, or what they look like. Some animals have a backbone. Animals with backbones are called vertebrates. Turtles, frogs, fish, birds and cats are all vertebrates. Animals without backbones are called invertebrates. Invertebrates include, worms, starfish, spiders and flies.

A fish lives its whole life in water. Like people, fish need oxygen to live. Fish use their gills to get oxygen from water. The water enters the fish's mouth, then moves through the gills behind the fish's head. The gills take in oxygen, and the water passes out.

A fish is cold-blooded. Cold-blooded means that the body temperature changes with the surrounding temperature. When a fish is in cold water its temperature will drop. When a fish is in warm water its temperature will rise.

An amphibian is also cold-blooded. It spends part of its life in water and part on land. Frogs and salamanders are both amphibians.

A reptile lives on land and has a waterproof skin. Examples of reptiles include lizards, turtles and snakes. A reptile is cold-blooded. Reptiles need the sun to warm their bodies.

A bird has a beak, feathers, two wings, and two legs. Birds are the only animals that have feathers. Feathers keep birds' bodies smooth so that they can fly or swim easily.

A mammal is an animal with hair or fur. A female mammal feeds its young with milk. Gorillas, whales, and cats are all examples of mammals. A mammal may have thick or thin fur or just a few hairs. Dolphins and elephants have just a few hairs. Mammals are warm-blooded. Their bodies stay at the same temperature, even when it's very cold or very hot around them.