Consider this scenario. The instructor of one of your classes promises you that there will be a surprise quiz next week. The class in question meets only on Mondays, Wednesdays, and Fridays, and a "surprise" quiz is one which, walking into the classroom, you have no idea whether it will be given on that day. The school week ends on Friday, so there are only three possible dates for the quiz: next Monday, next Wednesday, and next Friday. If you came to class on both Monday and Wednesday, and on both days, there were no quizzes, then you would know, walking into class on Friday, that there must be a quiz, for Friday is the last remaining day in which the instructor could give a quiz as promised that week. Therefore, it would not be a surprise for the instructor to give a quiz on Friday. Because Friday has now been eliminated, Monday and Wednesday are the only possibilities left for the surprise quiz. If you came to class on Monday and were not given a quiz, then Wednesday...