A turtle's shell is actually part of its spine and its rib cage, and the turtle can't crawl out of it even if it wanted to. It's not made out of bone, but of keratin, the same material that makes up human nails, antelope's antlers and horse's hooves.
The shell is also full of air chambers, which makes it a bit less onerous to carry around. Turtles such as leatherback turtles have shells covered with leathery skin.
There are three different types of animals in the turtle family. The plain turtle lives much of the time in water, and sea turtles spend nearly all of their time in water. Female sea turtles leave the ocean to lay their eggs on the beach, and other turtles might leave the sea to lie in the sun for a while. Freshwater turtles leave the water more often and can be found basking on logs or rocks.
Tortoises are found on land, even in deserts. They dig burrows, and when it's too hot, they go inside them to keep cool. The Galapagos tortoise is the largest tortoise. It can be 5 feet long and weigh close to 550 pounds.
Terrapins are found on land and water, but they always need to live near water, usually around swamps.