I need a brain teaser for D&D?

Something like "Two guys are guarding two doors. One leads to freedom, one leads to death. One man always tells the truth, one always lies. Choose the correct door."

Not THAT brainteaser, because everyone knows it. But a brainteaser I could incorporate into D&D that will require my players to talk and think.
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I'm love the riddles, guys! Keep em coming!
I need a brain teaser for D&D?
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