Bats are the only mammals that truly fly. While some other mammals, such as flying squirrels, are able to glide, bats alone use flapping wings to power their flight. Their wings are formed from modified front legs, with a membrane connecting their fingers together but leaving the thumb free. Extra-long hand and finger bones give them a large wingspan. Thin, lightweight bones keep their body weight low. Bats flap their wings using muscles in their chests and backs, as well as extra muscles in the wing membrane. The rear ends of bats are also modified for flight. The wing membrane connects to the hind legs, and sometimes even to the tail, giving the bat more surface area for flying. Strong toes, some with tendons that can lock into a rigid position, allow bats to hang upside down. Suspended from sharp claws, they can quickly take flight.