User: What makes airplanes fly?
Assistant: When an airplane engine starts, the propeller creates thrust and pulls the airplane forward.  As the airplane moves forward, more and more air moves over the wing.  The airplane wings are designed to be curved over the top and flat across the bottom.  As a result, the air molecules take longer to travel over the top of the wing than they do on the underside.  This creates a differential in air pressure between the top and bottom of the wing.  The difference in air pressure is called Lift.  As thrust increases and pulls the plane forward, the lift eventually increases enough to overcome the weight and drag and the airplane will take flight.