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While independently conceived and popularized by Kenton Knepper in his marketed Card-in-Wallet effect, “Kollosal Killer”, this strategy was first conceived of by Spain's Mago Anton in 1980. However, their applications of the idea are somewhat different in intent. In “Kollosal Killer” the “off by one” ploy is used as an out that may or may not be required to conclude the effect. In Anton's application, the “off by one” ploy is always trotted out to create an amusing twist that brings the effect to an unexpected successful finish. Anton included his trick using the ploy in a 1981 lecture at the national convention in Santander. He called it “El juego de la balanza” (The Scale Trick). The effect was this: The performer shows a blank card on which he draws a scale, the scale's tongue displaying “0”. This card is then signed, placed face down on the floor, and then the spectator stands on it. The performer looks at his assisting spectator and tells him that he is going to make an “adjustment”, by writing a prediction on a slip of paper, which is placed aside. The spectator is informed that he needs a dime to activate the scale. The performer pockets this coin, explaining that if the scale is right, he keeps the money; if it is wrong, he'll give the spectator back twice the amount. The card is turned over and it now reads “70 kilos”, but the spectator says he weighs only 65 kilos. The performer apologizes, gives back twice the money, but then has the spectator look at the prediction, which reads. “We are sorry to inform you that the scale is off by 5 kilos!”
Anton eventually released a marketed version of this trick at the 2000 FISM convention in Lisbon. The ad copy (translation by Roberto Giobbi) reads:
Mago Anton, in his untiring search for new effects and original presentation ideas, has transposed those feared “few extra pounds” into an amusing and surprising magic effect.
Effect: You give out a blackboard for examination, write a secret message on it and put it in an envelope, which is given to a spectator to hold.
Then you draw a set of scales that reads 0 kg on a flip pad. You invite a spectator up onto stage and get him to sign the picture (for obvious reasons women are generally more worried about those few extra pounds than men, so make use of this fact!). Stand the spectator on the drawing of the scales and tell him that he must tell the truth: for the first time everyone will know the truth and the whole truth about his weight!
The spectator gets off the scales and everyone sees that it reads 70 kg. With a great smile and sigh of relief the spectator announces that you have made a mistake. Surprised you ask what is (according to him) his real weight. Let us suppose that he says 62 kg. Draw everyone's attention to the envelope that the spectator has had all the time. Remove the blackboard and read out loud the message, “The scale is off by 8 kg.”
You do not need to be a genius of mathematics to work out that 70 minus 8 equals 62, exactly the weight the spectator claims to be! You reap you applause and the spectator goes back to his seat happy that he still keeps his greatest secret.