a kind of riddle using a play on words, e.g. 'When is a door not a door?' - 'When it's ajar.'

puzzle

a question, person, thing, circumstance, that is puzzling

mystery

something of which the cause or origin is impossible to explain or understand

puzzle, secret

a question, statement or description, intended to make a person think hard in order to know the answer or meaning; to make many holes in a person, thing

brain-teaser

a narrow passage of water connecting two seas; trouble; difficulty

crisis, difficulty

state of being uncertain; something which is uncertain

doubt, changeableness