Three logicians enter a restaurant. The waiter asks, 'Does everyone want coffee?' 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 everyone wanted coffee?
The first two logicians wanted coffee, but could only speak for themselves; if either did not want coffee, they would have answered 'No'.
If the first logician did not want coffee, he would know the answer to 'does everyone want coffee' is 'No'. Saying 'I don't know' revealed he wanted coffee. The second logician deduced this and, wanting coffee himself, also said 'I don't know'. The third logician wanted coffee and deduced that the first two did as well, allowing him to answer 'Yes'.
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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