By Margo Anton.

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 on a flip pad which reads 0 kg. You invite a spectator up onto stage and get them 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 them that they must tell the truth: for the first time everyone will know the truth and the whole truth about their 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 her) her real weight. Let us suppose that she 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 scales have an error of 8 kg”. You do not need to be a genius of mathematics to work out that 70 minus 8 equals 62, exactly the weight that the spectator claims to be!
You reap you applause and the spectator goes back to their seat happy that they still keep their greatest secret.