Mark goes to a doctor and says he has a problem with sex.
"Doc, I think my dick is just too damn small," he says.
The doctor asks him which drink he prefers.
Well, German beer," Mark replies quite bemused.
"Ah, there's your problem, it shrinks things, those silly German beers.. you should try drinking Guinness.
That makes things grow."
Two months later Mark returns to the doctor with a big smile on his face.
He shakes the doctor by the hand and thanks him.
"I take it you now drink Guinness?" asked the doctor.
"Oh no, Doc," replies the man, "but I've got the wife on German beer!"