To demonstrate Bernoulli's Principle.
Insert a drawing pin through the centre of a cardboard piece and place a cotton reel over the point of the pin. Then hold the cotton reel in one hand and the card in the other, and blow air down through it as hard as you can and release the card. The cardboard does not fall but remains stuck to the cotton reels as long as you blow air. The is due to Bernoulli's principle which was discovered by Daniel Bernoulli, a eighteenth-century Swiss scientist.
This principle states that when a gas or liquid is in motion, its pressure is reduced and the faster the motion, the more the reduction in pressure. Blowing into the cotton reel causes air to flow rapidly over the top of the card, lowering the air pressure. The stronger air pressure under the card keeps it from falling down. This same principle is what happens when an airplane moves forward. The wings are made so that air flowing across their upper surfaces at a faster rate than it rushes past the lower surfaces and due to Bernoulli's principle this lowers the pressure more above the wings than below, producing the lifting force that keeps the airplane flying.