MAN: I'd like to buy some dog food.
CHECKOUT LADY: Do you have a dog?
MAN: Yes.
CHECKOUT LADY: Where is he?
MAN: He's at home.
CHECKOUT LADY: I'm sorry, I can't sell this dog food to you unless I see the dog. Store policy.

The next day, the man returns.
MAN: I'd like to buy some cat food.
CHECKOUT LADY: Do you have a cat?
MAN: Yes.
CHECKOUT LADY: Well...where is he?
MAN: He's at home!
CHECKOUT LADY: Sorry, I can't sell this cat food to you unless I see your cat.

The next day the man returns.
CHECKOUT LADY: What's in the sack?
MAN: Put your hand inside.
CHECKOUT LADY: Hmmm...It's warm and moist! What is it?
MAN: I would like to buy some toilet paper.