After her junior year at MIT, Rebeca Eun Young Hwang '02, MEng '03, weighed summer-job offers from major investment banks. But Hwang, a Korean-born chemical-engineering major who'd grown up in Argentina, passed them up for an internship at a small pharmaceutical company near Mumbai, India. There, she witnessed scenes of everyday life that would change her view of the world and the kind of work she planned to do.

"I was fascinated by the contrasts between opulence and poverty on the trains, in the streets, and in people's homes," says Hwang, who was surprised that despite the poverty, most people seemed contented. "I remember watching small children playing in polluted water, and drinking from it--water that would make most people sick just to look at. I still carry those images with me."

She realized then that she wanted to use her technical knowledge to improve conditions for the world's poorest communities. Today, Hwang, 29, lives far from the developing world--in San Francisco, where she is a cofounder of YouNoodle, a startup that has attracted attention for its algorithm that predicts a startup's chances of success. But even now, she says, problems such as water quality are never far from her mind.

Hwang saw the importance of clean water firsthand at the age of 11, when she watched her mother drop bacteria-killing bleach into the family's water supply in Buenos Aires. (Her parents had emigrated from South Korea when Rebeca was six, seeking a less crowded, "bolder," and less constrained culture for their children.) For two years, they repeated the bleach ritual each day to avoid the cholera that was then ravaging much of South and Central America.

After she returned to MIT from India and completed her bachelor's degree, Hwang enrolled in a master's program in the Department of Civil and Environmental Engineering, where she studied with Susan Murcott '90, SM '92, a senior lecturer known for her efforts to bring cleaner water to the developing world. She soon headed to Nicaragua to lead a study that monitored use of a water-purification system for poor households in four rural villages. There, just north of Managua, Hwang's group used household filters donated by a nongovernmental organization to help people filter their own water. She was surprised by the results: the technology worked, but it didn't actually solve the problem. Water flowed through the filters so slowly that people sometimes resorted to the unfiltered supply. What's more, animals and children often played with the faucets, recontaminating the filtered water. "I faced the realization then that technology was not the answer to everything," she says. "It was a wake-up call for me that social, economic, and educational factors have to be aligned with the technology in order to make a real difference."

That realization was particularly jarring because science and math had always been the focus of Hwang's life. She remembers learning to count at a very early age and then ranking first out of 2,500 students for five consecutive years at her secondary school, the Colegio Nacional de Buenos Aires. She developed a taste for competition early on, winning Argentina's National Chemistry Olympiad as well as first prize in a national cancer-research contest.

At 15, Hwang read biographies of famous scientists such as Marie Curie and plotted the course of her life. "I wanted strongly to have an impact on the world," she says. She observed that the most exciting scientific research was coming from the United States and, more specifically, from MIT. So she set her sights on the Institute, remaining steadfast even after her parents and the Korean ambassador warned that she would never get in. When she received a letter of acceptance in March 1998, she recalls, "it was the first time that I had ever seen my father cry."