Wondering if folk have particular favourites amongst the many puzzles in our repertoire. I'll start the ball rolling with the 12 coins puzzle, reproduced below for convenience. The 12 Coins Puzzle You have 12 coins, one of which is counterfeit. It is either heavier than the other 11 coins, or lighter than the others - you do not know. Using only three weighings on a two-pan balance scale, how would you determine which is the counterfeit coin?