So there's this convent that, true to their mission, is situated right in the inner city. Over the years the neighbourhood has become the haunt of Ladies of negotiable affection and their clients.
Noticing this the nuns decide upon a plan to raise funds.
They post cards around the neighborhood reading "come get screwed by the sisters of mercy". Then they place a little red light above their door and wait.
Presently there is a knock on the door and, when the sister novice opens it, a very pent up looking bloke standing there.
"Where do I go?" he demands
The sister novice directs him to head up a flight of stairs. At the top is a tiny wizened nun with a cash box.
"How much?" demands the would be John
"$75" the old wrinkly bride of Christ informs him
The man eagerly hands over his money and the pious old dear places it in the cash box.
"Where do I go?" the bloke demands again
With a gnarled finger the old lady nun indicates that he should head down a different flight of stairs. So he does as directed.
At the bottom of the stairs is a very pretty nun standing by a doorway. The man, very pleased with what he sees sidles up to her.
Before he can say anything, the pretty nun tells him that he must go through the door.
Excitedly he does so.
Only to find himself back on the street on the other side of the convent. The door slams shut behind him and he discovers there's no way to open it from the outside. There is however s little sign in beautiful copperplate handwriting that reads: