So, the guy says to his doctor, "Doc, it hurts when I do this." And the doctor says to the guy, "Then don't do that."
OK, it's silly, old-fashioned humor (attributed alternately to Henny Youngman and to the 1920s vaudeville duo Smith and Dale). But you know what's even sillier, and arguably more old-fashioned? The exchanges that happen in many real-life doctors' offices today.