[Gandalf touches the stone surface of the walls of Moria]
Gandalf: Now, let's see... Ithildin. It mirrors only starlight and moonlight.
[moonlight shines on the wall, and the doors are revealed]
Gandalf: It reads: The doors of Durin, Lord of Moria. Speak, friend, and enter.
Merry: What d'you suppose that means?
Gandalf: Oh, it's quite simple. If you are a friend, you speak the password, and the doors will open.