Turtles, tortoises, and terrapins all belong to a group of four-legged reptiles that have hard outer shells, scaly skins, and horny beaks. Most people use the three words—turtle, tortoise, and terrapin—interchangeably. Scientists, however, sometimes make this distinction: a turtle is a sea reptile; a tortoise is a land reptile; and a terrapin is a fresh-water reptile.

It is correct to call all three turtles. They all breathe air through lungs and have shells that are made up of a “bony box” covered with horny plates or with soft skin. These shells are divided into two parts. One part covers the back; the other covers the underpart of the turtle’s body. Through the openings between the two parts, the turtle can thrust out its head, neck, tail, and legs.