The Cups & Balls is the oldest and most famous trick in magic. The performer shows three cups to be empty, and places them on the table. Giving one cup a tap with his wand he lifts it to find a small ball underneath. The ball is vanished only to re-appear under another cup! The ball is replaced under the cup, but a magic "tap" causes it to vanish and appear under a totally different cup. Another ball is discovered and vanished and then another. On lifting the last cup all three balls are found beneath it. Finally the cups are lifted to find an onion, lemon and potato beneath the cups! Many variations of routines are possible and available in standard books on magic.
The cups are (approx.):
Concave bottom to hold balls.