This gal walks into the drugstore and tells the pharmacist she wants to buy some arsenic. He says, "What do you want to do with arsenic?"
She said "I want to kill my husband because he cheats on me with another woman." The pharmacist replies: I can't sell you arsenic so you can kill your husband lady, even if he is cheating on you.
So she reaches into her pocket and pulls out a picture of her husband with the pharmacist's wife.
The pharmacist then says "Oh I didn't realize you had a prescription."