A man walks into a pub and orders a pint of beer and a pasty.
“How much will that be?” he asks.
“Nothing, Sir, it’s on the house.”
A little later, he orders another beer and again is told it’s on the house. After a third pint, he questions the barman.
“Why are all the drinks free today?”
“Oh, it’s quite simple really, Sir,” replies the barman. “The owner of this pub doesn’t know that I know he’s upstairs with my wife. So I’m doing to him down here, what he’s doing to me up there.”