The scientific principle that explains why airplanes fly is known as Bernoulli's principle. This principle states that an increase in the speed of a fluid (air, in this case) occurs simultaneously with a decrease in pressure. Airplane wings are designed to create differences in airspeed above and below the wing, leading to lower pressure on top and higher pressure below. This pressure differential generates lift, allowing the airplane to ascend into the air. The balance of lift, weight, thrust, and drag is central to the mechanics of aviation.