Hwang had spent just two days in the United States before arriving in Cambridge, and her English was limited. But she quickly picked up the language (her third--she's fluent in Korean and Spanish) and began to thrive academically. As a junior, she helped develop a device to make cholera treatment accessible to more people: a clamp used to regulate intravenous delivery of saline solution. The invention, which was less expensive and much easier to use than the existing technology, generated three U.S. patents.

With two MIT degrees in hand, Hwang headed to Stanford in 2003 to begin PhD studies in the Interdisciplinary Program in Environment and Resources. "I was still thinking about water purification, but not only from a technology point of view," she says. Drawing on both her technical expertise and her cultural understanding of Argentina, she set out to change the way water is delivered to residents of the underserved outskirts of Buenos Aires.

Growing up, Hwang had never realized that the water in the city's sprawling slums is contaminated with nitrates and arsenic at up to 10 times the levels considered safe. Left to fend for themselves, these communities--where more than half of the population lives on less than $2 a day--have formed coöperatives to pump treated water into homes. Using social-­network theory, Hwang analyzed these coöperatives to identify the most successful leaders and the practices that yield the most efficient distribution of clean water. She found that major players frequently make decisions in informal settings, such as soccer matches. By studying these often hidden networks and using software to map them, she uncovered successful patterns as well as weak spots. Hwang's fieldwork contributed to a national survey of water coöperatives aimed at improving service.

Hwang's Stanford advisor Leonard Ortolano, a professor of civil and environmental engineering, lauds what he calls her strong sense of social purpose and the deep quantitative and analytic skills that he says reflect her MIT background. "Rebeca is an extraordinary person and stands out even in the crowded world of outstanding Stanford doctoral students," he says.

While busy with PhD fieldwork, Hwang still made time for science and technology competitions, including the California Clean Tech Open and Stanford's BASES Social E-Challenge, which award funding to top social and entrepreneurial ventures. As a longtime contestant in such competitions, and then a judge and advisor, she saw a lot of good ideas but few ways for students to share and compare their ventures. "For a long time, there was no easy way to showcase projects from universities," she says. Frustrated by this situation, Hwang embraced an opportunity to do something about it. Never fazed by risk (her hobbies include skydiving, rugby, and whitewater rafting), she took a leave from her PhD program and joined Bob Goodson and Kirill Makharinsky, who'd met as students at the University of Oxford, to found YouNoodle in late 2007.

Named for the expression "use your noodle," YouNoodle functions in part like a LinkedIn for startups, aiming to foster innovation by providing a forum for students and entrepreneurs worldwide to connect and share ideas. Meanwhile, its much-discussed prediction algorithm seeks to forecast the success of a new business on the basis of information about its concept, finances, founders, and advisors. "It's all about finding clear patterns and applying technology to real-world problems," Hwang says.

Crammed into one room of a former restaurant in San Francisco's South of Market district, the startup has the look and feel of a college dorm lounge, complete with pool table and exercise equipment. With a database of 50,000-plus startups, it's developing quantitative tools that venture capitalists and others can use to find and assess the world's most promising entrepreneurs.

Hwang is committed to finishing her doctoral thesis and continuing her work with poor communities, but she is clearly determined to stay with her startup until it takes off. Part of YouNoodle's appeal, she says, is its ability to help students with innovative business ideas. "I've always seen myself as an entrepreneur and a risk taker," she says. "Right now I'm trying to help other young innovators accomplish their dreams."