riddle

/ˈrɪdəl/

a difficult problem

Chinese meaning: 谜；谜题；难题

The ancient riddle about the sphinx has puzzled scholars for centuries.

关于斯芬克斯的古老谜题困扰了学者们几个世纪。

Scientists are trying to solve the riddle of how the universe began.

科学家们正试图解开宇宙起源之谜。