An old man came to the doctor complaining of a very strong pain in his leg.
“I am afraid it’s just old age”, replied the doctor, “there is nothing we can do about it.”
“That can’t be; you are wrong!” said the old man.
“How can you possibly know I am wrong?” asked the doctor.
“Well it’s quite obvious,” the old man replied, “my other leg is fine, and it’s the exact same age!”