One stormy Friday morning, Philosophy Teacher Doug Kremm announces to his Philosophy of Happiness Class that he already has a date in mind for a surprise quiz next week. On Sunday night, the terrified students convene to predict which day the surprise quiz would take place. They quickly realize that if the quiz was on Thursday, then there would be no quiz on Tuesday. In that case, the students can infer that the quiz is on Thursday by the end of Tuesday. But if they know it would happen on Thursday before it actually happens, then it would no longer be a surprise. And since Dr. Kremm never lies, the students conclude that the quiz must be on Tuesday.
“But then,” a student says, as the others prepare to leave, “if we know that it’s definitely on Tuesday, would that quiz still be a surprise?”