Children might think that the difference between birds and other kinds of animals is that birds have wings, but students in the fourth grade learn otherwise -- insects and bats have wings, too! The difference between birds and all other creatures is actually their feathers. It doesn't even matter whether or not the animal can fly; if it has feathers, it is a bird. Fourth graders also learn that birds can grip their perches with their claws, even while they are asleep. Birds are not the only animals with beaks or bills, but all birds have them. There is no such thing as a bird with teeth.
Fourth-grade students learn that mammals live in all different kinds of environments. They live underground, in the air, in the water and on land, and they all have hair, even whales and dolphins. Almost all mammals have seven bones in their necks, no matter how long their necks are. A mouse and a giraffe both have necks with seven bones. There are only approximately 4,000 species of mammals, which is not a lot compared to the 800,000 kinds of insects there are in the world.
Fourth-graders learn that reptiles are cold-blooded, meaning their blood heats up and cools down based on the temperature around them. Some reptiles have unusual survival abilities. For example, some species of lizards have tails that come off when a predator catches them. The lizard runs away and then grows a new tail. Chameleons are a type of lizard that can change color to match their surroundings. Snakes can open their mouths wider than their heads and swallow animals that are bigger than they are.
Fourth-grade students learn that amphibians are cold-blooded like reptiles. Amphibians are all born in the water, and when they are babies, they are similar to fish. They have tails for swimming, and they breathe through gills instead of lungs. When they get older, though, they grow legs and lungs so they can function on land. Some of them also shed their tails. Amphibians use several different organs to breathe, including gills, lungs, their skin and the lining in their mouths. They live water-lives and land-lives both at once, which is why they are called "amphibians." The word means "two lives."