The Unexpected Hanging
A prisoner who has committed a heinous crime is before a judge. The judge sentences the prisoner to death by hanging, but adds a cruel twist to the sentence (the prisoner’s crime is particularly heinous). The prisoner is to be hanged on one of the following seven days - but it must be a surprise which day it is. The prisoner is not allowed to know.
Returning to his cell the prisoner is a bit disturbed at the prospect of being hung without knowing when (presumably it worse when you don’t know what day you are to die) - and confides his fear to his lawyer. His lawyer tells him not to worry.
“Look,’ he says with a smile. “They can’t hang you at all now. The judge has made it a condition that you must be surprised. But think about it. If you make it to Saturday without being hung then Sunday is the last day they could do it. But then it wouldn’t be a surprise would it? So that makes Saturday the last day they could possibly hang you. But hang on - if Saturday is the last possible day then it also can’t be a surprise to hang you then. So that make Friday the last possible day - and so on back through all the days of the week. They can’t possibly hang without breaking the judge’s orders."
The prisoner is comforted by this line of reasoning and stops worrying about the prospect of being hung at all. When suddenly on Wednesday, much to his great surprise, he is taken from his cell and hung.
This is a great puzzle, not so much because we have two different kinds of legitimate reasoning in conflict with one another, but because one strand of reasoning is shot down by reality itself. It seems a catch 22. If the prisoner realizes the "solution," then he rests easy in his knowledge and is taken by surprise anyway.
I love stuff like this. Totally pointless, but a nice little brain warmup.