Dean Kamen, 55, is probably best known for inventing the two-wheeled Segway people mover. But his latest creation promises to move mountains. The device, dubbed the Slingshot, can transform the filthiest water into pure, drinkable H2O. The 300-pound, electric-powered, dishwasher-size prototype purifies both freshwater and saltwater, basically by vaporizing, compressing, and condensing the liquid. At Deka Research & Development, his Manchester, New Hampshire–based company, Kamen's standing challenge is this: Bring him the vilest goo and he'll run it through the Slingshot, knock it back, and ask for more.

Outside: How much faith do you have in this machine?
KAMEN: Last summer we took one into Tiananmen Square, in China, and fed it local river water. It was not only brown and disgusting; it was gelatinous. It had stuff bobbing around in it that would almost defy description. And out came the cleanest, purest water you've ever had. We were standing around handing it out to people in paper cups.

How did you get interested in water in the first place?
We at Deka were designing home-based dialysis systems—which require water clean enough to inject— so we were very interested in how to purify local water. Even in the developed world, tap water can have arsenic, cryptosporidium, lead, and other contaminants, so we spent millions trying to figure out everything that could possibly go wrong with water and how to fix it. Eventually I said, hey, we could help a few hundred thousand people—or we could expand this technology to serve a billion people around the world.

Did everybody immediately say, "Great idea!"?
Everybody said "You're nuts!" But I thought, If we can manufacture a machine that has no need for chemicals or activated charcoal or membranes—and if we can create something small and rugged enough to carry into remote areas and have it run for years with no maintenance—we'd solve the problem.

You recently tested a prototype in Honduras, where people were getting sick from drinking dirty river water. How did it go?
From day one we were making a thousand liters a day. People loved it. Now we're ready to go with more prototypes in other villages next year. In a few years we hope to have them widely available. In very high volume, they'd be under a few thousand bucks apiece.