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A pain in the leg
An old man goes to his doctor, complaining about a pain in his leg that doesn't heal, and wants a diagnosis and explanation. The doctor checks out his leg, but can't find anything wrong, so he gives the old guy a full physical exam, and still can't come up with any possible explanation for the pain.
The doctor hands the patient his bill and says, "I'm sorry, but the pain in your leg is simply caused by old age; there's nothing I can do about it."
The old man replies with a look of disbelief, "That's impossible! That can't be!"
The Doctor says, "What do you mean? I'm the expert here; if you know so much, how can you say it's NOT old age?"
The patient answers, "I'm no doctor but it doesn't take a medical degree to tell that your diagnosis is wrong. Clearly you're mistaken. After all, my other leg feels just fine."
"So what?" says the doctor "What difference does that make?"
"Well it doesn't hurt a bit, and it's the SAME AGE!"