Conundrum

   
A conundrum is a difficult question or riddle, especially one using a play on words in the answer; a difficult choice or decision that must be made.

‘The word “conundrum”,’ Preston repeated helpfully, ‘When you say the word, doesn’t it look in your head like a copper-coloured snake, curled up asleep?’ (Terry Pratchett – “I Shall Wear Midnight”)