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