puzzler
noun [ C ] informal uk/ˈpʌz.əl.ər/ us/ˈpʌz.əl.ɚ/
令人困惑的事物，难解之事
something that is difficult to explain or understand
I don't know what happened to the money - it's a real puzzler. 我不知道那笔钱去哪儿了，真是怪事。
Synonyms: puzzle teaser (PROBLEM)