CENTER CITY (WPVI) -- There's something deliciously charitable happening right now in Center City Philadelphia and it's pretty simple.
At Rosa's Fresh Pizza, a slice is always a dollar - and if you are feeling generous, you can buy a piece or two for someone in need.
The concept is so popular, and people are paying it forward in such a big way, they are feeding thousands of the city's needy.
"| mean, where do you find a dollar pizza, first off, and if I can help a person in need with just a slice of pizza?" said Brianna Presume of Northeast Philadelphia.
But that's exactly how it works at Rosa's on 11th and Chestnut.
Last year, Mason Wartman left his finance job in New York City and moved back home to open the pizzeria he named after his mother.
"I saw the success of the dollar pizza stores in Manhattan, and I love the simple elegant business model of 'do one thing and do it well,'" Wartman said.
One day, a customer asked if he could buy a slice for someone in need.
"I ran out and got a Post-It and wrote a smiley face on it and stuck it up back there - and used Post-Its to keep track of the system," said Wartman.
Nine months and more than 8,000 slices later, Wartman now keeps track of the charity on the register.
Generous people still leave notes, mostly to grace the walls with kind sentiments like 'You are beautiful' and 'You can do it.'
Other notes signify the gratitude.
"This guy wrote on a paper plate "God bless you. Because of you I ate off this plate, the only thing I ate all day," said Wartman.
Ralph Niglio says if it weren't for Rosa's, he wouldn't have had anything to eat this day.
"This means a lot because I am going to have something in my belly," he said.
And Wartman treats every pizza patron the same.
"They deserve to eat in the store and enjoy the music as much as any other paying customer would," he said.
It's a gesture that, Harold Brown says, means a lot more than you might think.
"It helps people. They come in, they are hungry, they can sit down and eat," he said.
And customers say it feels good to know exactly how they are helping.
"Sometimes you give them stuff and you don't know what they are going to do with it. But to come in here and know you can buy a slice for yourself and buy a slice for them, you know you are feeding somebody," said John Rodriguez.
Wartman also sells t-shirts and sweatshirts that buy even more slices. Inside the shirts he's sewn in resources for the city's homeless - from shelters to computer classes.
Customers pay it forward with pizza in Center City