a confusing problem or question that is very difficult to solve 令人迷惑的难题；复杂难解的问题 The role of clouds is one of the big conundrums of climatology. 云的作用是气候学的一大难题。
a question, usually involving a trick with words, that you ask for fun 谜语
Word Originlate 16th cent.: of unknown origin, but first recorded in a work by Thomas Nashe, as a term of abuse for an eccentric or pedant, later coming to denote a whim or fancy, also a pun. Current senses date from the late 17th cent.