A pattern of air flow demonstrating the Bernoulli principle. A wing-shaped object placed in a wind tunnel acts as an airfoil, where the upper surface of the object is more curvaceous than the lower surface. The air (horizontal lines) flowing over the wing has to travel a greater distance in the same amount of time as the air flowing beneath it. The Bernoulli principle states that the internal pressure of a gas is lowered the faster it travels. The pressure exerted downwards on the wing by the air above it is less than that exerted upwards from below. This difference can lift a wing upwards, making flight possible.