<human>:  When a plane flies, what is the difference between the pressure and the density of the air?
<bot>: The pressure of the air is the force per unit area. The density is the mass per unit volume. The pressure and density of the air is related to the speed of the plane. The higher the speed, the higher the pressure and the lower the density. This means that the air will be compressed in front of the plane and expand behind the plane. This difference in pressure is what causes lift. The faster the plane flies, the more lift it gets.