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# Duplicating Dynamo's domino demonstration

## by Peter W McOwan, Queen Mary University of London

*
TV magician Dynamo baffled the audience on his show by being able to
predict in advance which numbers would be on the ends of a random chain
of domino pieces. Want to amaze people with the same trick?
*

## Doing the domino deception

The trick here is a little known mathematical fact about a full set of dominos. You can place all the domino pieces in a line, matching the numbers on the tiles, and this line will in fact join up making a complete cycle. The actual order of the pieces will vary but if you have a full set you can always find a chain that will join with itself. Knowing this it's easy to see how the trick works. Before you show the trick you remove one domino and hide it. Write the numbers on this tile down as your prediction. But don't use a double domino, e.g. 3 spot and 3 spot. The spectator won't notice one piece is missing but when they are challenged to put all the dominos in a single chain, the end of their chain will be missing that link, and the numbers will match your prediction.

## Domino but different

Now you know the pattern that makes it work, you can transfer this basic idea for a prediction trick by using your own cycle of data. For example, rather than the two numbers on a domino, one of the attributes of your piece could be a colour and the other a letter say. Or a piece could have just text on it like an artist's name with one of their songs for a music top 40 type presentation. Just make sure that at the beginning you write down the full cycle on paper before you transfer the numbers, letters, songs or whatever, to individual pieces.

It's a bit like reusing computer code: perhaps reskining code written to be the high score table in a video game to be used by a reality dance TV program to keep track of the leader board. Taking a pattern and mapping its properties to another application is something that computer scientists do all the time, and this example is only one of many where a magician will take a basic mathematical idea and creatively transform it to accomplish what seems like a different magical effect.

## Doing Domino digits

We end with a simple little mathematical domino trick that's can be used with a younger audience. I leave you to work out how it works. Ask a volunteer to secretly pick a domino from a facedown set, then ask them to do the following calculations. Multiply the number on one end of the tile by 5, and then add 7. Take this result and multiply by 2, then add the remaining unused number on the domino tile. The minute they tell you the answer, secretly subtract 14 from the result. The 2 digit number you have left will comprise the two numbers on the selected domino ... ta ra ... take a bow.