Do you want a receipt with that?

Sometimes when I buy small food items such as a candy bar, soda, or bag of chips the clerk will ask me if I want a receipt. I always tell them; "The only way I'm returning this is if I throw up in your store, and then I won't be expecting a refund." Sometimes they laugh nervously. Sometimes they laugh sincerely. Sometimes there is an awkward silence. Most often they say something like "Well, we don't want that to happen, do we?" I don't know... Maybe I should stop saying that.