Noun

• S: (n) riddle, conundrum, enigma, brain-teaser (a difficult problem)
• direct hypernym / inherited hypernym / sister term
• S: (n) problem (a question raised for consideration or solution) "our homework consisted of ten problems to solve"
• S: (n) puzzle, puzzler, mystifier, teaser (a particularly baffling problem that is said to have a correct solution) "he loved to solve chessmate puzzles"; "that's a real puzzler"
• S: (n) case (a problem requiring investigation) "Perry Mason solved the case of the missing heir"
• S: (n) homework problem (a problem that students are assigned to do outside of class)
• S: (n) riddle, conundrum, enigma, brain-teaser (a difficult problem)
• S: (n) poser, stumper, toughie, sticker (a particularly difficult or baffling question or problem)
• S: (n) Gordian knot (any very difficult problem; insoluble in its own terms)
• S: (n) koan (a paradoxical anecdote or a riddle that has no solution; used in Zen Buddhism to show the inadequacy of logical reasoning)
• S: (n) pons asinorum (a problem that severely tests the ability of an inexperienced person)
• S: (n) rebus (a puzzle where you decode a message consisting of pictures representing syllables and words)
• derivationally related form