What Makes an Animal a Reptile?

Step 1 of 5

Skin is the first half of the answer.

A reptile is covered in dry scales of the same stuff as your fingernails. They overlap, they hold water in, and they let a reptile sit in the open without drying out.

Step 2 of 5

The egg is the other half, and it is the bigger one.

A reptile egg has a leathery or hard shell, its own water sealed inside, and its own food. It is a private pond the parent can leave anywhere dry.

Step 3 of 5

So the group covers some very different animals.

Lizards, snakes, tortoises and crocodiles all belong. They look nothing like each other and share the same two rules.

Step 4 of 5

Reptiles take their warmth from outside.

None of them heat themselves from within. They move into sun or shade to set their own temperature, which costs them almost nothing in food.

Step 5 of 5

Two of the rules bend, and the group still holds.

Some snakes and lizards keep the egg inside and give birth to live young. Birds, meanwhile, came from inside this group — which makes them reptiles too, on the family tree.

A tortoise's shell, a snake's lack of legs and a crocodile's jaw are all built on the same reptile plan underneath.