"I'd have to go with Denmark," Campos says. "Sending a postcard abroad costs an astonishing 50 krones, or about $7.30." Where do I find postcards and how do I choose them?

The class has received postcards from as far away as India, Scotland, Ireland, Denmark, Hawaii, and Egypt, along with 33 U.S. states. Each time a postcard arrives, students mark the location on a ...