Are you pulling my leg?

An old woman went to the doctor complaining of a terrible pain in her leg. “I am afraid it’s just old age,” replied the doctor after examining her. “There is nothing we can do about it.” “That can’t be,” fumed the old woman. “You don’t know what you’re doing.” The doctor countered, “How can you possibly know I am wrong?” The old woman replied, “Well it’s quite obvious— my other leg is fine, and it’s the exact same age!”