For all the tourist money and eye-rolling that Segways have brought to D.C., few of us know anything about the guy who designed the thing: Dean Kamen, a New Hampshire-based serial inventor. His other claims to fame, if we afforded engineers the same adulation as we do sports stars (a discrepancy Kamen derides in SlingShot), would include the portable dialysis machine, the first wearable insulin pump, and a wheelchair that lifts its user to eye level with people standing on two legs. Kamen’s latest innovation is a relatively compact water distiller that, through a heat-recycling scheme called vapor compression, uses less energy than a standard hair-dryer. SlingShot, which shares a name with this new machine, is part biography, part case study on how Kamen is scaling up a design that could help solve world water insecurity. Part of the fun of SlingShot is that it’s a porthole into the life of a genius. Kamen is plenty eccentric—his closet contents consist of nearly 100 percent denim—and he doesn’t seem to have time for (or interest in) many personal relationships. His musings on the fleetingness of life and confidence in the face of mind-boggling obstacles are vaguely motivational. But the 90-minute doc starts to drag after the first half-hour, in part because the director’s uncritical admiration of the inventor makes the film feel a little bit like an advertorial. I was left with some unsettling questions about Kamen’s funding plans and his quid pro quo partnership with Coca-Cola, a company that makes money by convincing people to drink something other than water. SlingShot offers a hopeful glimpse of a rare kind of capitalist that uses his powers for good, but a drop or two of skepticism would have given the outcome a cleaner aftertaste.