A man is about to go into a bar when he is accosted by a nun. She says "why are you going into that evil place, to drink alcohol? It will cripple your body and rot your mind!"
The man says "I just want to have one drink. Besides, how would you know about what alcohol does to anybody? Have you ever even had a drink?"
"Certainly not!" cried the nun.
"Then how would you know anything about it? I dare to take a drink, then you can lecture me all you want."
The nun thinks it over for a while and finally says "Okay, I'll have a drink, but just to show you how terrible it is. But I won't be seen going in that place with you. Just get a measure of gin in a coffee cup and bring it out to me so no one will know I'm drinking alcohol."
So the man goes in the bar and orders a Guinness and a shot of gin in a coffee cup.
The bartender says "Is that damn nun out there again?"