The Mines of Moria Riddle
While the Hobbit had many riddles in it, the central riddle in the Lord of the Rings saga involved the group getting into the Mines of Moria. Speak, Friend, and Enter!
Those words - "Speak, Friend, and Enter" - were enscribed over the hidden doorway into Moria. The group sat for hours trying to figure out just what password should be spoken. It was only after quite some pondering that they realized that all they had to do is say the word FRIEND and they could get in!
You can use many variations of this in your own adventures. It could be something like
Speak Softly to Enter
Speak Loudly to Get In
State Your Name to Gain Admittance
and the code word would be "Your Name".
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