This mathematical trick uses the metaphor of playing the piano to colourful effect
Method: The magician invites a volunteer to sit at a his table and to place his hands as if playing the piano.
He then places two cards between the third and little finger of the volunteers left hand saying: 'Here's one pair'. Then another two cards between the third and second fingers saying:'Here's another pair'. Next the magician puts two cards between the first and second fingers saying:
And here's another pair' Lastly, he places two cards between the volunteers forefinger and thumb saying: 'And yet another pair here as well'.
The magician does the same with the volunteers right hand except this time he puts only one card between the thumb and forefinger stressing: 'but this one is an odd card'.
The performer repeats what he has done always stressing that cards were placed in pairs. He then continues in the same fashion only this time he removes each of the pairs of cards from the volunteers left hand, separates them and places them side by side on the table - again stressing: 'Here's a pair'. He continues with the helpers right hand pairs until he reaches the one odd card which he hands to the volunteer asking him to place it on top of either of the two piles.
This done, the magician taps the pile on which the volunteer placed the odd card and says that the odd card will magically fly across to the other pile. He then picks up the pile which is supposed to have the extra card added and separates the cards into side by side pairs saying each time once again: 'Here's a pair'.
The volunteer sees that there are four pairs of cards and the odd card has apparantly vanished! The magician separates the other pile in the same manner and the volunteer sees that there is an odd card! - the illusion is complete - the odd card must have jumped across to the other pile!
How the Trick is Done
This trick works because of course earlier in the trick the volunteer is holding 4 pairs of cards in the left hand and 3 pairs and one odd card in the right. The cards are then divided into two piles of seven cards which the volunteer doesn't notice because of the emphasis on pairs. When the odd card is added to a pile it turns it into an even numbered pile!