Bernoulli's Principle states that an increased speed of a fluid causes low pressure. A wing on an airplane is shaped in such a way that when it passes through air, the air that passes above the wing must travel a distance farther than the air that passes below the wing. In order to do so in the same amount of time, the air above the wing must pick up speed, thereby creating low pressure. This low pressure above the wing and high pressure below the wing causes lift, which allows an airplane to fly.
