This is related to a mathematical/philosophical problem. Unexpected hanging paradox Say Gomez is told he will be tested 3 times. Suppose he reaches the 3rd to last day of the season with no tests as yet. It's a certainty that he'll be tested that day (and the next two). But no - it's supposed to be a surprise! Therefore, it can't be that day. So roll the logic back to the day before that. It can't be that day either, for the same reason. The chain of logic continues, and eventually he can't be tested on any day. Of course, the Wikipedia page goes on to point out just how very surprised Gomez will be, when he gets tested on some random day anyway, despite his airtight logic!