First, get permission from a grown-up to use a hair dryer.
Turn your hair dryer on cool and point it towards the ceiling.
Put the ping pong ball in the stream of moving air. Watch -- it just floats there.
Tip the hair dryer to the right or left and the ball still stays in the stream of air.
You're probably wondering why the air doesn't just blow away the ball. The air from the hair dryer is blowing right on the ball and pushing it up. But did you know that gravity from the earth is also pulling the ball down? That's why the ball isn't blown right up into space.
Do you know what keeps the ball from blowing off to the side? It's the fast moving air coming from the blow dryer. The ball doesn't fall because the air from the hair dryer is going really fast around the sides of the ball. This fast moving air pushes on the sides of the ball and keeps it from falling.
What happens when you try this with two ping pong balls at the same time? What happens if you use a different kind of ball? Try it at home.