The Cups and Balls is a classic magic effect.
The Magician shows three small plastic cups and three balls. One of the balls is placed on top of the center cup. The other two cups are stacked on top of the center cup. The trapped ball magically appears beneath the center cup. Now the next ball is placed on top of the center cup. Again the other cups are stacked on top. The second ball mysteriously appears under the center cup along with the first ball. This is repeated and finally, the third ball magically convenes with the first two balls under the center cup.
Very easy to perform.