LAST EDITED ON Aug-02-03 AT 09:02 PM (EST)
 
I do not know what reasoning you used on the guys, you can try this:

The guy picks a card. The probability of the card being the ace is 1/3. You show him one of the others being a king.

1. If he does nothing, you might have shown the king to pigeons (and not to him) with the same effect, the probability of his success (that his initial choice is the ace) is still 1/3. The probability of the opposite event (that the ace is the card he did not pick and you did not show) is 1 - 1/3 = 2/3, because the ace surely is somewhere.

2. He asks himself: What is the probability that the other card I did not pick is also a king? The same as the probability that the 2 cards I did not pick were both kings to begin with, that is 1/3. Probability of the opposite event (that the other card is the ace) is 1 - 1/3 = 2/3, again, because the ace surely is somewhere.

If you want just to convince the guys by an experiment, do it with something that does not cost them much, say nuts, pennies, etc.