Birds have them. So do bats and butterflies. Daedalus and Icarus donned them to escape Minos, king of Crete. We're talking about wings, of course, or airfoils, which function to give an aircraft lift. Airfoils typically have a slight teardrop shape, with a curved upper surface and a flatter lower surface. As a result, air flowing over a wing creates an area of higher pressure beneath the wing, leading to the upward force that gets a plane off the ground.
Interestingly, some science books invoke Bernoulli's principle to explain the uplifting story of airfoils. According to this logic, air moving over a wing's upper surface must travel farther -- and therefore must travel faster -- to arrive at the trailing edge at the same time as air moving along the wing's lower surface. The difference in speed creates a pressure differential, leading to lift. Other books dismiss this as hogwash, preferring instead to rely on Newton's tried-and-true laws of motion: The wing pushes the air down, so the air pushes the wing up.