A man was driving down the road when his car broke down near a monastery. He went to the monastery, knocked on the door and said: 'My car has broken down. Could I possibly stay the night?'

The monks graciously accepted him, fed him dinner, and even fixed his car. But as the man was drifting off to sleep, he heard a strange sound. The next morning he asked the monks what the sound was, but they said: 'We can't tell you. You're not a monk.'

The man was mystified but thanked them anyway and went on his way.

Some years later the same man broke down again in front of the same monastery. Once more, the monks fed him, fixed his car and allowed him to stay the night. Just as he was falling asleep, he heard the same strange noise that he had heard years earlier. The next morning he asked what it was, but the monks replied: 'We can't tell you. You're not a monk.'

The man was so frustrated that he said: 'Look, I'm dying to know. If the only way I can find out what that noise was, is to become a monk, how do I become a monk?'

The monks said: 'You must travel the earth and tell us how many blades of grass there are and the exact number of grains of sand. When you find these numbers, you will become a monk.'

The man set about this daunting task. For the next fifty years he traveled the length and breadth of the globe, counting blades of grass and grains of sand until his work was finally complete. Armed with this information, he returned to the monastery and announced: 'I have traveled the earth and have found what you asked for. There are 347,498,675,212,031 blades of grass and 664,981,732,434,109,597,436,501 grains of sand on this earth.'

The monks replied: 'Congratulations. You are a monk. We will now show you the way to the sound.'

The monks led him to a large wooden door and told him: "The sound is right behind that door.'

They gave him the key to the door and he opened it. Behind the wooden door was another door made of stone. The monks gave him the key and he opened it, only to find another door made of silver. He demanded the key to that door from the monks and was given it. As he opened the door, it revealed yet another door, this time made of copper. He asked for the key to the copper door, certain that this would reveal the answer to the secret that had troubled him for over half a century, but behind it he merely found another door, this time made of iron. On and on he went - through doors made of emerald, ruby and gold - until finally the monks said: 'This is the key to the last door.'

The man was hugely relieved. He unlocked the door, turned the knob, and behind that door he was amazed to discover the source of that strange sound.

But I can't tell you what it was because you're not a monk.