Prove it

Guy: I want to buy dog food.
Seller: Do you have a dog?.

Guy: Yes
Seller: Where is it?.

Guy: At home.
Seller: Sorry, i can't sell you dog food unless i see the dog first, it is our policy.

The Next Day....
Guy: Do you have cat food?.
Seller: Where is your cat?.

Guy: It is at home.
Seller: Sorry, i can't sell cat food for you unless i see the cat.

Two Days Later....

The Guy went there holding a nylon bag.

Seller: What is in your bag?.
Guy: Put your hand inside first. (The seller puts his hand inside)

Seller: It's cold, what is it?.
Guy: It is my shit! I need toilet paper