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An airplane flys because air moving over and under its wings travels at different speeds, producing a difference in air pressure, low above the wing and high above the wing. The lifting force, usually called lift, depends on the shape, area, and tilt of the wing, which causes the air streaming above and below the wing to travel at different speeds. The greater distance the air must travel above the curved upper part of the wing forces that air to move faster to keep up with the air moving along the flat lower surface. This sucks the wings upward, allowing the airplane to fly.