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To create lift, the air on the top of the wing is forced to speed up, and thus has a lower pressure than the air on the bottom. However, at the end of the wing, the higher pressure air will try to force itself above the wing, mixing with the lower pressure air, and creating what we call a vortex. These vortices reduce the angle of attack of a wing, create drag, and slow down the plane.

This is different from the circulation theory of lift, which is erroneous.