This is a dirty joke:

A little old lady went to a grocery store and picked up 2 cans of cat food but when she got to the checkout counter the cashier said, “I can’t sell you the cat food unless you can prove you have a cat because some older people buy this for themselves since it’s cheap.” So the little old lady went home in a huff then came back the next day with her cat and said, “See, I have a cat!” The cashier sold her the cat food.

A week later the same little old lady went to the store and got 2 cans of dog food but the cashier said, “I can’t sell you dog food unless you prove you have a dog since some older people buy this for themselves because it’s cheap.” Again, the little old lady went home in a huff but came back the next day with her dog and said, “See, I have a dog!” The cashier sold her the dog food.

The following week the same little old lady went back to the store carrying a small box with a hole in the lid. "Stick your finger in that hole then show me your finger and tell me what’s on it," the little old lady told the cashier. “Eww,” said the cashier, “it’s poop!” The little old lady smirked and told the cashier, “Now you have proof that I need to buy toilet paper!"