| [ QuizWit ] in KIDS 글 쓴 이(By): dalgong (달공아저씨) 날 짜 (Date): 2001년 8월 30일 목요일 오후 06시 40분 51초 제 목(Title): Surprise Test Paradox 이거 누가 설명좀 해주세요. 어디가 잘못 된 건지. The teacher says: "Next week you will have a surprise-test." The students reason: "If we will have the test on the last day of the next week (be it Sunday), then the previous day (Saturday) we could say: ‘There was no test either on any of the previous days, or today, so it’s for sure we will have the test tomorrow.’ But a test that we know about a day before is not a surprise-test any more, so it’s impossible to have a surprise-test on Sunday. "Days Monday through Saturday are left. If we will have the test on Saturday, then Friday we could say: ‘There was no test either on any of the previous days, or today, so it is for sure we will have the test tomorrow (Saturday) or the day after tomorrow (Sunday). But we have just established that a surprise-test is not possible on Sunday, so it is for sure we will have the test tomorrow.’ But a test that we know about a day before is not a surprise-test any more, so it is impossible to have a surprise-test on Saturday. "Days Monday through Friday are left. If we will have the test on Friday,…" The students reach the conclusion that it is impossible to have a surprise-test on any day of the week. In other words, it is impossible for the teacher to keep his/her promise. But if the test occurs on Wednesday, for example, there is no way the students could learn of it on Tuesday, so it is really a surprise-test, so the teacher keeps his/her promise. |