In the Einstein year 2005 I published a card game named 'The Einstein Code'.

The game is based on the mathematical pigeon hole principle.
The object of the game is to crack the opponent's code
consisting of 4 letters randomly chosen from X,Y,Z and T.

It was edited by Die Keure (Bruges) and sponsored by Texas Instruments.

Pigeon hole principle: if you have to put 10 pigeons in 9 pigeon holes, 
there will be at least one hole with 2 (or more) pigeons.

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Here is an other well known application of this principle.