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D_Malik comments on Open Thread, Apr. 13 - Apr. 19, 2015 - Less Wrong Discussion
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In case you didn't encounter it on Facebook, here is an excellent logic puzzle from Singapore:
Albert and Bernard just became friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates.
May 15, May 16, May 19
June 17, June 18
July 14, July 16
August 14, August 15, August 17
Cheryl then tells Albert and Bernard separately the month and the day of her birthday respectively, so Albert knows the month while Bernard knows the day.
Albert: I don't know when Cheryl's birthday is, but I know that Bernard does not know too.
Bernard: At first I didn’t know when Cheryl's birthday is, but I know now.
Albert: Then I also know when Cheryl's birthday is.
When is Cheryl's birthday?
Raymond Smullyan calls these sorts of puzzles (where characters' ability to solve the puzzle is used by the reader to solve the puzzle) "metapuzzles". There are some more examples in his books.