A turtle's legs may move slowly, but its neck can move with lightning speed. The neck muscles are extremely flexible, and the skin is very loose. This allows the turtle to pull its whole neck inside the shell in case of danger. Not all turtles can hide their heads inside their shells, however. Sea turtle shells barely cover the animal's back and provide no protection to the head or legs. And tortoises must protect their faces by hiding behind the tough scales on their front legs.