(Note: This is a long joke, but you really do have to read the whole thing to appreciate it. It won't take too long.)
There's a guy driving his car out in the boonies (I hope this is a fairly well-known expression) somewhere, and he gets a flat tire. He can't get the car started again, so he just starts walking down the road trying to find something. He comes up to a monastery and goes in. He asks the monks there, "Hey, can I borrow a cell phone or something so I can call someone to come fix my car?" They reply, "Sorry, we're monks; we don't have cell phones. But, our wagon is going into town the next day and you can get help there. We'll let you stay the night here." He agrees.
In the middle of the night he gets woken up by a loud, amazing, awe-inspiring sound, and he has no idea what it is. He tries to fall back asleep, but he keeps hearing it. He gets up because he needs to figure out what it is. He goes downstairs, goes through winding passageways, and he comes up to the biggest door he's ever seen, and the head monk is standing in front of it. He can tell the the sound is coming from right behind the door. He asks the head monk, "What's that sound?"
The head monk replies, "Sorry, I can't tell you; you're not a monk."
"Why not? Can't you just tell me what it is?"
"Sorry, I can't tell you; you're not a monk."
"Come on, I'm not going to be able to fall asleep if I don't know what that sound is."
"Sorry, I can't tell you; you're not a monk."
The man isn't happy, but he goes back to bed. He has trouble sleeping that night, and while getting ready to go he keeps asking the monks what the sound is, but they all reply with the same answer: "Sorry, I can't tell you; you're not a monk."
He goes back to town, gets his car fixed, and goes on with life. But he keeps hearing that sound in his mind, and after a few months he can't stand it any more. He finds the monastery again and goes back to the head monk.
"Hey, you remember me? I'm that guy that was here a few months ago. I just need to figure out what that sound is."
"Sorry, I can't tell you; you're not a monk."
"Well then, how do I become a monk?"
"Are you sure you want to become a monk? The journey is long and hard."
"Yes. I need to figure out what that sound is."
"Alright. First, you need to count all of the stars in the night sky."
He spends a whole year, counting and recounting, consulting astronomers, etc. He thinks he has the answer, and goes up to the head monk and says it. "Very good," he replied. "Now, you must count all of the grains of sand on the seashore."
He spends years and years, crawling on beaches all over the world, counting each individual sand grain, because he needs to figure out what this sound is. He finally has his answer, and goes to the head monk and says it. "Very good," he replied. "Now, you must climb the tallest mountain and see how small you are compared to the world."
He trains himself, gets all of the necessary equipment, and makes it up. By this point he has gotten quite a bit older and wiser. He goes up to the head monk and the head monk says, "Finally, you are ready to become a monk." They go through all of those twisting passageways, and up to the huge door. The head monk opens the door, and the man finally figures out what the sound is...
...but I can't tell you; you're not a monk.