Reptiles are vertebrates. That means they have backbones. There are four main kinds of reptiles. Snakes and lizards are in one group. Crocodiles and alligators are in another. A third group includes turtles and tortoises. The last only has one species. It is the tuatara. It is the last living member of a group of reptiles from the time of the dinosaurs.

Reptiles have scales to protect their bodies. Scales keep their skin from drying out. The scales can be hard or soft. They can be big or small. Some reptiles live on land. Some spend a lot of time in water.

Reptiles are cold-blooded. Unlike mammals, their bodies do not stay the same temperature all the time. If their bodies get too cold, they slow down. They need to sit in the sun to warm up. If they get too warm, they get out of the sun.

Most reptiles lay eggs. The eggs often have a leathery shell. A few kinds of reptiles give birth to live babies. A few reptile females do not need to mate with a male. They can lay eggs that grow a copy of themselves. When reptile babies are born, they look like little adults. They have to take care of themselves.

There are more than 6,500 kinds of reptiles. The heaviest is a kind of crocodile. It can weigh over 2,000 pounds (over 900 kilograms). The smallest is the dwarf gecko. It is about 3/4 of an inch (about two centimeters) long.