One of China’s small joke: proof of

A man saw a store selling discounted items, and went inside.
“You buy what?”
“I want to buy dog food.”
“We have provided here, you must prove your dog.”
“Where there is such a provision?”
“Discount merchandise is one such provisions.”
Man for a long time to negotiate with the salesperson, salesperson, or do not agree to sell him.
There is no way, men had to take the dog to bring home only to buy the dog food.
A few days later, the man went to the store to buy cat food.
“Give me two boxes of cat food.”
“We have provided here, you must prove your cats.”
Is salesperson that the man has negotiated with her for a long time, the result was forced to return home to her cat brought it to buy the cat food.
A few days later, the man holding dug a hole big boxes arrived at the store, find the sales.
“You buy what?”
“You reached into the cardboard boxes where the know.”
Stretched into the hands of the salesperson: “What is inside ah, sticky.”
“I want to buy two volumes of toilet paper.”