Having just learned about air and atmospheric pressure when we made a DIY barometer, this lesson served as a great extension.
All you need is a ping pong ball and a working hair dryer.
I had one son hold the hair dryer straight up, while it blew cool air at the highest setting. Our other boy gingerly moved a ping pong ball into the air stream.
WHOA! They were amazed to see the ball float in mid-air without falling. Even more fun is when they moved the hair dryer at a slight angle to see the ball still stay suspended!
Why it Works
The force of air from the hair dryer will push the ball up, and the force of gravity will push it down. It will stay suspended at the point where the force pushing up and gravity pulling it down are equal.
The reason the ball does not shoot out of the stream of air, is air pressure. The air coming out of the hair dryer has a lower air pressure than the air surrounding it. This keeps the ball within the column of fast-moving air coming from the dryer. This principle was discovered by Bernoulli in the 1700s, long before the first hair dryer was invented.