Synonyms and related words :
Chinese puzzle, baffling problem, brain twister, crossword puzzle, crux, enigma, enigmatic question, floorer, jigsaw puzzle, knot, knotty point, mind-boggler, mystery, nut to crack, perplexed question, perplexity, poser, problem, puzzle, puzzlement, question, question mark, sixty-four dollar question, sticker, stumper, tough proposition, vexed question, why