A woman walks into a pharmacy, strolls over to the counter, and catches the pharmacist's attention. "Can I please get some arsenic?" she asks.
"Arsenic? What do you want arsenic for?" asks the pharmacist. "It's for my husband," she replies.
"You husband?" exclaims the pharmacist, "I hope you don't mean what I think you mean!" She just nods. "Well, lady," he replies, "I'm an honest man. I can't sell you arsenic, I wouldn't if I could, and I don't know what made you think you could just stroll into a respectable store and expect me me to sell you arsenic.!"
She doesn't say a word. She just reaches into her purse, fishes out a photograph, and hands it across the counter. It is a picture of her husband, in bed with the pharmacist's wife.
Slowly the pharmacist looks up, over the counter, and then straight at her. "Lady," he says, "why didn't you tell me you had a prescription?"