conundrum
/kəˈnʌndrəm/noun
(pl. conundrums)
1- a confusing and difficult problem or question复杂难题：
one of the most difficult conundrums for the experts.
对专家们来说最难的难题之一。
1.1
- a question asked for amusement, typically one with a pun in its answer; a riddle（尤指答案中含双关语的）谜语。
词源
late 16th cent.: of unknown origin, but first recorded in a work by Thomas Nashe, as a term of abuse for a crank or pedant, later coming to denote a whim or fancy, also a pun. Current senses date from the late 17th cent.