A man walks into a bar and orders a whiskey.
When the barman serves it up, he takes it out to the bench in front of the bar to drink it.
As he's enjoying his drink, a nun walks by, and glares at him sourly. "How can you pollute your soul with the Devil's drink like that?" she asks.
The man shrugs. "It's not the Devil, it's just whiskey."
"But it's sinful and wicked!"
"How do you know it's so bad, then? Have you ever tasted whiskey?"
"Of course not! My sisters and mother superior told me how evil drink is."
"But how do they know? Have they ever had a drink?"
They go back and forth like this for a while, before at last, the nun relents. "Well, I suppose that if I were to try a sip of whiskey, I would better understand how it corrupts the soul. But it wouldn't do for any of my sisters to come by here and see me drinking. Could you order me one in a teacup?"
The man agrees this is fair, and walks inside to the barman.
"Two whiskeys, but put one in a teacup, please."

The barman slams his hand down on the bar and shouts, "Is that damn nun here again!?"