Reptiles share many traits with other classes of animals, but they are the only animals with the following combination of traits: skin covered with a sheet of scales; self-regulated body temperature; and young that look like miniature adults (most hatched from eggs, but some born live).
Scaly Skin Is In!
Reptiles have dry, scaly skin. But they don't need moisturizer! Their special covering actually helps them hold in moisture and lets them live in dry places.
Reptile scales are not separate, detachable structures -- like fish scales. Instead, they are connected in a "sheet," which is the outermost layer of skin. Every so often, this layer of skin is shed and replaced. In some reptiles the skin flakes off in chunks. In snakes, the skin is usually shed in one piece.
What about turtles and tortoises? You may not think of their shells as being scaly, but they are! They are complex structures made up of bones and scales that develop from the outer layer of skin. It's natural body armor!
Not Too Hot, Not Too Cold
Reptiles, like amphibians, are ectotherms (what used to be called "cold-blooded"). This means that they can't produce sufficient internal heat to maintain a constant body temperature. Instead, reptiles' body temperature varies, depending on the surrounding temperature. Though some reptiles can generate enough internal heat to raise their temperature for a specific purpose -- like female pythons brooding their eggs -- though they cannot maintain this temperature for long.
So reptiles are responsible for regulating their own body temperature. When it's cold outside and they need to warm up, they often bask in the sun to raise their body temperature. When it's too cold to even bask, reptiles may brumate. This means they're in a hibernation-like state, but they may have periods of wakefulness and even drink when necessary.
When it's hot outside, reptiles spend much of the time burrowing during the day, becoming active only at night.
At birth, young reptiles are miniature replicas of their parents, though their coloring may be different. In this, reptiles are very different from amphibians (like frogs and salamanders), which undergo a drastic change from larvae to adults.
Most reptiles -- including all turtles, crocodiles, and alligators -- lay eggs. But other reptiles -- about one out of five types of lizards and snakes -- bear live young.