A man is driving along the road and his car suddenly breaks down just outside a monastery. He knocks on the door, three monks greet him, and he asks, "My car broke down. Do you think I could stay the night?"
The monks graciously accept him, feed him dinner, even fix his car. That night, as the man tries to fall asleep, he hears a strange thumping sound at the end of the corridor. He gets out of bed, walks down the corridor to a locked wooden door. From behind the door he hears the strange sound: Thump! Thump! Thump!
He returns to his bedchamber to sleep. The next morning, he asks the monks what the sound was, but they say, "We can't tell you. You're not a monk." The man is disappointed but thanks them anyway and goes about his merry way.
Some years later, the same man breaks down in front of the same monastery (as they do in stories like this). The monks accept him, feed him, even fix his car. That night, he hears the same strange noise that he had heard years earlier. The next morning he asks what it was, but the monks give the same reply, "We can't tell you. You're not a monk."
The man says, "All right, all right. But I'm dying to know. If the only way I can learn about that sound is to become a monk, how do I become a monk?"
The monks reply, "You must travel the earth and tell us how many blades of grass there are and the exact number of beach pebbles. When you find these numbers, you will become a monk."
Such is his desperation that the man sets about his task. Forty-five years later, he returns and knocks on the door of the monastery. He says, "I have travelled the earth and have found what you asked. There are 145,236,284,232 blades of grass and 231,281,219,999,129,382 beach pebbles on the earth."
The monks reply, "Congratulations! You are now a monk. We shall now show you the origin of the sound."
The monks lead the man to the wooden door, where one monk says, "The sound is right behind that door."
The man reaches for the knob, but the door is locked. He says, "Real funny. May I have the key?" The monks give him the key, and he opens the door. Behind the wooden door is another locked door made of stone. The man demands the key to the stone door. The monks give him the key, and he opens it, only to find a locked door made of ruby. He demands another key from the monks, who provide it. Behind that door is another locked door, this one made of sapphire. So it went until the man had gone through doors of emerald, silver, topaz and amethyst.
(Pause: So there are some clues: wood, stone, ruby, sapphire, emerald, silver, topaz, and amethyst. Can you work out the punchline?)
Finally, the monks say, "This is the last key to the last door." The man is relieved no end. He unlocks the door, turns the knob, and behind that door he is amazed to find the source of that strange sound.
But I can't tell you what it is because you're not a monk.