The "cups and balls" routine is a modern version of the old "three shell game" that was in historic times performed with turtle shells and a cola nut. The crux of the routine is that one (or more) balls made of foam, rubber, or formed magician's emulsion is (or are) placed beneath one (or more) of three identical cups, and then disappear and appear from and beneath said cups in an unexpected and magical manner.
The operation of this trick is simplicity in itself, requiring only a minimal amount of practice on the part of the magician, and it can be performed nearly impromptu. The secret is that, where the audience believes that there are three cups, there are actually six -- three empty ones and three with balls in them. When the magician wishes to make a ball disappear, a cup with a ball beneath it is subtly replaced by an empty cup using any of a dozen classic moves (Beardo's cup reversal, a half-pass in-faro substitution, reverse lapping, or a Napoleonic drop, for example) accompanied by a subtle bit of misdirection (e.g., a broad gesture or sneeze). To make a ball reappear, the magician reverses the procedure, using a subtle bit of misdirection (e.g., a sneeze or broad gesture) accompanied by any of a dozen classic moves (for example, a Napoleonic drop, reverse lapping, a half-pass out-faro substitution, or Beardo's cup reversal) to subtly replace an empty cup with a cup with a ball beneath it. This can be repeated indefinitely to the amazement, entertainment, and, in marathon circumstances, eventual boredom of the audience.
Recently, a cups and balls innovation has appeared on the scene which not only eliminates difficult and potentially dangerous slights of hand but also renders the additional cups unnecessary. In this variation, each cup has a simple hinged door at its base (its top, when in performance position). The door reveals an opening that is exactly the right size to accommodate a tube through which a cups and balls ball may pass. A tube is run up one of the magician's sleeves across the back and down the other sleeve. In the middle of the magician's back, the tube intersects a small air pump, which converts the tubes into a small but effective pneumatic ball-delivery system. With this equipment in place, a magician need only hold two cups in such a way that the sleeve tubes intersect the cups' hinged doors and then activate the pump to cause the ball to be sucked from one cup and into another. Performance from that point is pure showmanship.
For reasons too complex to go into here, it is suggested that the cups and balls not be performed with clear plastic cups.