A woman walks into a pharmacy and asks for some arsenic poison.
He says, "What do you want with arsenic?"
She replies, "I want to kill my husband because he's cheating on me with another woman."
The pharmacist says, "I can't sell you arsenic so you can kill your husband, lady. Not even if he is cheating on you with another woman."
So she reaches into her pocket and pulls out a picture of her husband in bed with the pharmacist's wife.
The pharmacist says, "Oh, I didn't realise you had a prescription."