Effect: You give a spectator a blackboard to examine, you write in it a secret message and you introduce it in an envelope that you deliver to a spectator. Then, in a sketchpad, you paint a scale that marks 0 kgs. You ask a spectator to go on stage and sign the scale (for reasons that are well known, they tend to be more worried than them about those “extra kilos”, so take advantage of it!). Place the spectator standing on the scale and warn her that with you there are no tricks: for the first time everyone can know the truth and nothing but the truth about their weight!
The spectator lowers the scale and everyone can see that it marks 70 kilos. With a huge smile and a sigh of relief, your spectator will announce that you have made a mistake. Surprised, ask him what his weight is (according to her). Let’s say it says 62 kilos.
You direct your attention to the envelope that you gave to a spectator and you show and read aloud the message that it contains: “This scale has an error of 8 kilos”. You do not have to be a maths genius to know that 70 minus 8 are 62, precisely the number of kilos that the spectator says weigh!
You collect your deserved applause and the spectator returns to her happy seat to continue hiding one of her best kept secrets.
