The Deciding Magic Domino Trick

Layout any set of Western double-square dominoes facedown on your playing surface.  Ask your subject to shuffle them and then to think of a number between 6 and 12.  You then draw the subject's given number of tiles from the boneyard and set them aside, acting as if you are concentrating very hard on which tiles you chose.  Now ask your subject to draw three tiles from the ones you have set aside.  Now you concentrate again and draw one of the three and place it prominently away from all the others.  Now ask your subject to layout all the remaining dominoes in the conventional way with same-numbers touching same-numbers, and in a roughly circular sequence. 

Once they have placed their last tile, you turn over the previously selected domino and, voila, it magically turns out to be the exact one to complete the circular sequence of domino tiles.

The secret is that any Western domino set will always completely link back to itself, and it doesn't matter which domino you select as the deciding one.  The selection process is just there to dress up the trick and make it seem mysterious and skilful.

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