The scientific principle that explains why planes can fly is Bernoulli's Principle, along with Newton's Third Law of Motion. Bernoulli's Principle states that as the speed of a fluid increases, its pressure decreases. In aviation, the shape of an airplane wing, or airfoil, causes the air to flow faster over the top surface than the bottom. This pressure difference creates lift, allowing the plane to ascend. Additionally, when a plane pushes air downwards (as per Newton's Third Law), an equal and opposite reaction lifts the plane upwards. Together, these principles enable flight.