Reptiles (meaning "to creep") are a group of animals that have scales (or modified scales), breathe air, and usually lay eggs. The term reptile is loosely defined in everyday English to mean scaly, cold-blooded, egg-laying animals. In cladistics (a way of classifying life forms), the reptiles are more strictly defined and include the descendants of the most recent common ancestor of the turtles, lepidosaurs (lizards, snakes, tuataras), and archosaurs (crocodilians, dinosaurs, and birds). The maintenance of body temperature (cold- vs. warm-blooded) is not a factor in this classification, but skull and egg structure are.