They’re called winglets, and their purpose is to reduce turbulence at the tips of an airplane’s wings. The air pressure on the bottom of a wing is greater than the pressure on top, so when air flowing across the two surfaces meets at the wing tip, it forms a vortex-a miniature tornado. The vortices created by a large airplane are strong enough to flip a smaller plane that is following too closely.

By breaking up vortices, winglets reduce the drag on an airplane, which translates into fuel savings. So why don’t all airplanes have winglets? The airflow around winglets is complicated, so designing them is tricky. It’s easier to improve an airplane’s lift-to-drag ratio by simply making the wing longer, though this can lead to other problems, such as fitting into gates.

Conventional upright winglets are currently used on a number of airliners, including the Boeing 747-400 and the Airbus A330 and A340. On some of its 737 models, Boeing uses “blended” winglets, which curve up from the wing instead of sticking straight up.

Blended winglets have a counterpart in nature: Some birds curl the tips of their wings upward to reduce drag while gliding.