Say you want a slice of pizza, but you don’t have enough money. At Rosa’s Fresh Pizza in Philadelphia, Pennsylvania, you can “pay” with a post-it note.

The process works like this: A customer who has cash buys a slice of pizza for $1. If they want, they can pay an extra dollar and get a sticky note. Someone in need can trade the sticky note in later for a slice of pizza.

Rosa’s came up with the idea in 2013. Now, customers treat 224 people in need each day to a slice of pizza.