This guy goes to a grocery store and asks the clerk behind the counter
for two cans of dog food.  
"Do you have a dog?" asked the clerk.
"Yes I do!" replied the puzzled customer. 
"I'm sorry sir" said the clerk "but you're going to have prove to me that you have a dog before
 I can sell you dog food." 

Back home went the frustrated customer to get his dog and pulled it
on its leash all the way back to the store. 
"Here's my dog!" weezed the tired customer. 
"Thank you sir, here is your two cans of dog food." 

Two days later the guy returns to the same store and goes up to the same clerk and
says:
 "Two cans of cat food please." 
"Do you have a cat sir?"  "Of course I do!" said the exasperated customer. 
 "I'm sorry sir but I have to see your cat before I can sell you cat food." 

The guy storms out of the store, goes home, grabs his cat, drags it back to the store and 
holds up the cat by it's tail for the clerk to see. "

Thank you sir here is your two cans of cat food."  
The very next day. The guy returns to the store, approches the clerk and places on the counter 
a white shoebox with a small hole on the cover.

"Yes sir", asked the clerk, "what can I do for you?"  
"Put your finger in the hole" ordered the customer. 
 "I beg your pardon?" said the clerk. 
"Do as I say!" ordered the guy.  

Cautiously the clerk slid his finger all the way in the hole.

"Pull it out and tell me what it looks like!" said the guy "It looks like SHIT!" said the disgusted 
clerk; to which the customer replied

"THAT'S RIGHT!!, Now give me two rolls of toilet paper!"