The campus resident dog, Negro, in Columbia is a quick study and figured out that people go to the campus shopkeeper to pay for cookies with green paper.
One day, he arrived with a leaf in his mouth and as a reward for his ingenuity, the shopkeeper gives him a dog safe treat.
https://www.goodnewsnetwork.org/clever-dog-always-brings-a-leaf-to-buy-himself-treats-at-the-store/