Photo: Hug It Forward

Four years ago, Zach Balle had a successful real estate career in Phoenix, which earned him an impressive paycheck but left him unfulfilled. "There was a sense that I'd made it," he says, "and yet I couldn't ignore this empty feeling in my stomach." After a colleague offered some unorthodox advice—"Book a flight to a country you've never been to"—Balle found himself in a small Guatemalan community where many children received their lessons outdoors. "If it rained, they didn't have class that day," says Balle, now 28. "I decided I wanted to build them a school—which was totally unrealistic. But I knew if I could figure out a way to include the townspeople in the project, we could make it happen."

Armed with newfound inspiration, Balle quit his job and started researching his plan. He was dismayed to discover that even a simple structure would cost nearly $15,000 for supplies and labor. When he explained his dilemma to a contact in the Peace Corps, she told him about a method of construction she was using that transforms trash into building material. Balle decided to help her build a school in the Guatemalan community of Granados. His friend Heenal Rajani, 31, who had been casting about for a more meaningful endeavor, decided to help out as well. After local children collected empty soda bottles and stuffed them full of chip bags and candy wrappers, the resulting "eco-bricks" were placed between chicken wire panels and covered with cement to create the walls of the structure.

Their two-room schoolhouse, completed in October 2009, used more than 5,000 plastic bottles and 2,053 pounds of trash, cost less than $6,000 to build, and now serves roughly 300 of Granados's students. In 2010 Balle, Rajani, and three other friends, including Joshua Talmon, 31, officially established the nonprofit Hug It Forward to fund more eco-brick schools across Central America; so far they've built 17 in Guatemala and one in El Salvador. The San Diego–based organization, which finances the school projects partially through eco-tourism trips (volunteers can sign up at ServeTheWorldToday.com), now publishes a free online manual to help others replicate their model elsewhere around the world. "Being a global citizen isn't about swooping in as a superhero," says Talmon. "There are more wins if we all work together."