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.
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