Aluminum Cups and Balls

You announce that you are about to perform one of the oldest effects in all of magic. In your left hand you hold three cups. On the table you have placed three balls. All you ask of your audience is that they watch carefully.

One-by-one you place the cups upside down in a row on the table. With your thumb and forefinger you place one of the balls atop the center cup. You stack the first and third cup on the center cup trapping the ball between the cups. You make brief eye contact with your spectators, snap your fingers, and tap the stack of cups smartly. You lift the stack. To the amazement of your spectators, the ball has travelled down through the stack of cups and sits mischievously on the table.

Now your spectators know what to look for. 
Again you place the cups on the table one-by-one, but this time you place the center cup over the first ball. You place the second ball atop the center cup. You stack, snap and tap. With an intrigued look you lift the stack to reveal that the second ball has travelled down through the stack of cups and is sitting on the table next to the first ball.

Without hesitation you heighten the effect by repeating the procedure with the third ball. Your spectators are by this time convinced that there is something funny about those cups. They demand to inspect the cups and the balls. No problem. You shrug and hand them over.