Discuss your favorite close-up tricks and methods.
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I am looking for a reference to a trick which is probably an old mathematical trick or, maybe, just a con. Forty pennies are arranged on a table in a square with each side consisting of groups of one, two, three, and four. The spectator is allowed to add one penny at any spot, the magician moves several coins around and shows that each side still consists of 10 coins. This is repeated several times. It seems to me that Martin Gardner has written about this but I am, so far, unable to locate it. Thanks for any help.
There's a version in Richard Neve's "Merry Companion" (1716), but in Michael Weber's "Life Savers" (also referenced in "Knack Magic Tricks") it's stated that the principle for this has been traced back to Van Etten's "Mathematicall Recreations" (1633). It probably goes even further back because Van Etten's book first appeared in French in 1624 and was itself mainly a compilation from even earlier sources.