Three logicians walk into a café. The server asks, 'Does everyone want lemonade?' The first logician says 'I don't know.' The second logician says 'I don't know.' The third logician says 'Yes!' How did the third logician know that all three wanted lemonade?
The first two logicians wanted lemonade but didn't know if the others did; if either didn't want it, they would have answered 'No'.
If logician 1 or 2 did not want lemonade, they would have answered 'No' immediately. Their answer 'I don't know' signaled 'I want one, but I don't know about you.' The third logician also wanted lemonade, so hearing two 'I don't know' responses meant all three wanted one.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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