riddle:
n. 谜语;神秘事件;(筛分土石的)粗筛
v. 在……上弄许多小洞;充斥,布满;用粗筛筛;摇动(炉栅)使灰落下;出谜;为(某人)解(谜)
Can you answer this riddle?
你能猜出这个谜语吗?
This is the riddle you have to solve.
这是你必须要解决的谜题。
They had to do with the riddle of his existence.
他们必须解开他的存在之谜。
mystery:
n. 秘密,谜;神秘,神秘的事物;推理小说,推理剧;常作 mysteries 秘技,秘诀
The play is a murder mystery.
这出戏说的是一桩神秘的凶杀案。
The meeting was cloaked in mystery.
会议笼罩着神秘的气氛。
She's a lady of mystery.
她是一个神秘的女人。
He's a bit of a mystery.
他这个人有点儿神秘。
riddle意为"谜,谜语",是用来猜的;而mystery意为"奥秘,神秘事物",是用来探索的。