A man is driving along the road one rainy night and his car suddenly breaks down just outside a monastery. He knocks on the monastery door, monks greet him and he asks, "My car broke down. Do you think I could stay the night?"
The monks graciously welcome him in, feed him dinner, offer him a bed to sleep in and even fix his car while he sleeps.
During the night, the man is awakened by a strange thumping sound outside his room. He gets out of bed, walks down the corridor to a wooden door. From behind the door he hears the strange sound: Thump! Thump! Thump!
Curious, he turns the handle of the door, but the door's locked. So he returns to his bedchamber to sleep.
The next morning he goes down to the dining room for breakfast and asks the abbot what the sound was. But the abbot replies, "Sorry, but I'm not allowed to tell you. Only monks are able to understand what the noise means and what causes it." The man is disappointed but thanks the abbot and brothers for their hospitality anyway, and goes about his merry way.
Exactly one year later, the same man's car breaks down in front of the same monastery (as they do in stories like this), knocks on the door, is welcomed, fed, and offered a bed for the night. In the middle of the night he hears the same strange noise that he'd heard the previous year, gets up, tries the door at the end of the corridor but it's still locked. In the morning he asks again what the noise was, but the abbot gives the same reply, "I can't tell you. You're not a monk."
And the very next year, his car breaks down in front of the same monastery, enters, eats dinner, goes to bed, awakes from the noise. And again the next morning asks what it was, and is told, "Sorry, 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 abbot replies, "You must show extreme patience. For example, travel the earth and return to tell me how many blades of grass there are and the exact number of pebbles on the beach. When you find these numbers, come back here to tell me and we will accept you as a monk."
Such is his desperation that the man sets about his task. Forty-five years later, he returns and knocks on the monastery's door. "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 abbot shouts in glee, "Congratulations! You are now a monk. I can now show you the origin of the sound."
He leads the man to the wooden door and says, "The sound is right behind this door."
The man reaches for the knob, but the door's locked. "Real funny. May I have the key please?" The abbot gives 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 abbot gives him the key, and he opens it, only to find a third locked door encrusted with rubies. He demands another key from the abbot, who provides it.
(Pause: So there's the clues: wood, stone, rubies. Can you work out the punchline?)
Finally, the abbot says, "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.