A cork is removed from a small brass can and out onto the table drops a yellow pea, a black-eye pea, a green pea and a black pea. The spectator is asked to select one of the peas. The performer asks: "Would it be magic if the only thing that comes out is the pea you selected?" So, all of the peas are placed back into the can and sealed by the cork. The spectator begins to say outloud as instructed "I want my pea!" After the performer waves his or her hand over the can, the cork is removed and out into the spectator's hand is poured warm water (which is mistaken as pee by the spectator).
This is just one effect. The device can be used to exchange other small objects. This is a terriffic that you'll love to carry in your pocket.