Reptiles are vertebrates, they have scaly skin that keeps their bodies from drying out, their young do not go through a larva stage like amphibians but instead look like small versions of the adults when they hatch. Reptiles are ectotherms, so they must bask in the sun or find a warm spot to get warm and become active, and they must find shade or a cool spot to cool off. In cold conditions they become sluggish and don't move around much, and some enter a state of torpor or hibernation if it will be cold for a long time.
There are four main groups of reptiles: turtles and tortoises; lizards and snakes; crocodiles and alligators; and the tuatara, the only species left from an ancient group of reptiles that goes back to the dinosaurs. Some reptiles spend most of their time in water, and many spend their time on land. Reptile species can be found in all types of habitats except polar ice and tundra.
Most reptiles make nests or dig holes to lay their eggs in. Some, like crocodiles and alligators and some species of snakes, then stay to guard the nest and even help the hatchlings get started in life. But most mother reptiles leave the nest once the eggs are laid. The hatchlings are independent from the start and must find their own food and shelter.
There are more than 6,500 known species of reptiles. The heaviest is the saltwater crocodile, weighing up to 2,000 pounds (908 kilograms). The smallest is the dwarf gecko, measuring only three-quarters of an inch (16 millimeters). The longest snake is the reticulated python, at up to 33 feet long (10.5 meters). The largest lizard is the Komodo dragon, weighing up to 175 pounds (80 kilograms) and measuring up to 10 feet long (3 meters). Tortoises have the longest life span in the reptile order; some can live more than 150 years.