Scientists classify the billions of animal species on Earth into groups based on characteristics like whether they have a backbone, how they reproduce, or if they are warm or cold-blooded. The main groups are mammals, which are warm-blooded and feed their young milk, reptiles that are cold-blooded and lay eggs, and amphibians that can live both on land and in water. Animals are also classified as either vertebrates, which have backbones, or invertebrates, which do not.