Lift the tubes to show a glass under one and a bottle under the other.  Lower the tubes over the bottle and glass and when you lift them, THE BOTTLE AND GLASS HAVE CHANGED PLACES! 

The tubes are replaced and the bottle and glass switch back to their original positions.  During the process the tubes can be shown empty. 

Having done this once or twice something goes wrong and you show  TWO bottles and NO glass.  Trying to hide the error, you try to cover the bottle but THIRD bottle appears. Each time you try to fix the mistake, another bottle appears until YOUR TABLE IS OVERFLOWING WITH BOTTLES! (eight to be exact).