Three logicians walk into a cafe. The server 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?
If either the first or second logician did not want coffee, they would have answered "No". Saying "I don't know" meant they wanted coffee but didn't know the others' choices.
The question asks if *everyone* wants coffee. If Logician 1 did not want coffee, he could definitively answer "No". Saying "I don't know" reveals he wants coffee. Logician 2 reasons the same way and reveals she also wants coffee. Logician 3, wanting coffee himself, now knows all three want coffee and answers "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: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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