Bats are mammals of the order Chiroptera (/kaɪˈrɒptərə/; from the Ancient Greek: χείρ - cheir, "hand" and Ancient Greek: πτερόν - pteron, "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, can only glide for short distances. Bats do not flap their entire forelimbs, as birds do, but instead flap their spread-out digits, which are very long and covered with a thin membrane or patagium.