Things need :
§ Ping pong balls
· Place the ping pong balls a short distance a part on a table.
· Use the straw to blow air between them. The ball move towards each other.
Air presses on all things. This is called air pressure. When you blow between the ping pong balls, you blow away the air that is between them. This means that air is no longer presses against one side of each ball. But air still presses against the other sides of the balls, and that air pressure pushes the ball together.