Show the audience that you have three ping pong balls in your hand.
Cover them with your other hand.
Rub your hands back and forth.
Take away your hand, and all see that one of the ping pong balls has disappeared.
The secret is rather simple. From the start, you have not three balls, but two and a half!
While rubbing your hands together, you press the half-ball onto the whole one.
Only be careful at the end to show the ping pong ball from the side of the half-ball – or risk having your secret exposed.