ABSTRACT
The argument that there can be no surprise exam is an example of a backward induction argument. Despite this argument, a surprise exam is obviously a possibility, and not simply for pupils deficient in rationality or memory. We must suppose the pupils are completely rational and not suffering from any defects of memory, and that they know they are rational and have good memories. There is no paradox otherwise. That the unintelligent and the forgetful can be surprised gives rise to no logical puzzle but is a psychological commonplace.
