A man walks into a local store...

and attempts to buy an enormous amount of cat food. The cashier is not used to seeing this much cat food being purchased and worries that the man might sell the cat food for profit, so he demands that the man show him proof that he has cats. The man goes home and returns with his cats, and the cashier gives him his cat food.

The man returns after some time, this time attempting to buy an enormous amount of dog food. The cashier says, "I thought you had cats! I'm going to need some proof that you have dogs."

So the man returns with his dogs and the cashier has no choice but to give him his dog food.

After a few weeks, the man returns with a large sack. He shows it to the cashier, who immediately recoils and says, "What the hell is this!? You can't just walk in with a bag of shit!"

The man replies,

"I needed toilet paper."