Philadelphia has the highest obesity rate of America's big cities. It also has an ambitious plan to turn 632 corner stores into greengrocers. But whether that will have a health impact remains to be seen. Temple's Center for Obesity Research (CORE) is working with the city to study how shopping habits change when healthy options are introduced. "I don't think we know much about how well this works," says CORE Director Gary Foster. "It's a field in its infancy…nobody has really done it at such a big scale."