This is an old chestnut- though usually it involves a condemned man who is told that he is to be executed one day in the next week- and that he will not know, in advance, on what day he will executed. Well, he obviously cannot be executed on the last day- if he were not executed before, he would know that this was the only day left so he would know he was to be executed that day. But, given that he knows the he cannot be executed on the last day, he also cannot be executed on the day before! etc. Under these conditions, he can never be executed. So he is completely surprised when he is executed on the third day!
