A woman walks into a pharmacy and asks the pharmacist for some arsenic.
Surprised, the pharmacist asks, "Madam, what do you want with arsenic?"
The woman replies that she wants to kill her husband.
Horrified, the pharmacist says, "Madam, I can't sell you arsenic to kill a person!"
The woman reaches into her handbag, takes out a photograph and lays it down on the counter. The photo shows a man and a woman in a sexually compromising position. The man is her husband. The pharmacist recognises his wife as the woman in the photograph.
The pharmacist picks up the photo and nods, "Ah, madam, I didn't realise you had a prescription."