“With a suddenness that startled them all the wizard sprang to his feet. He was laughing! "I have it!" he cried. "Of course, of course! Absurdly simple, like most riddles when you see the answer." Picking up his staff he stood before the rock and said in a clear voice: Mellon! The star shone out briefly and faded again. Then silently a great doorway was outlined, though not a crack or joint had been visible before. Slowly it divided in the middle and swung outwards inch by inch, until both doors lay back against the wall.” GandalfMoriaDoors Of Durin Book:THE FELLOWSHIP OF THE RING Source: THE FELLOWSHIP OF THE RING