From the outside, the water purifier looks like a black box. It's about the size of a dormitory refrigerator. Inside, there's a system for purifying water that's actually quite old and common. Drug companies use the same method to purify water for use in medicines [source: MECO]. The U.S. Navy has used the method to desalinate drinking water [source: MECO].

Drug company and submarine versions aren't practical for developing countries, though. They're too big to move and need technicians on call. The Slingshot is simpler and more portable.



All of these purifiers work by vapor compression distillation. Kamen once ran down a partial list of what this process can purify: the ocean; water laced with arsenic, poison, heavy metals, viruses and bacteria; liquid at a chemical waste site; or the contents of a latrine [source: Comedy Partners]. Remarkably, all it takes is boiling and re-liquefying water at precise temperatures. Let's see how it works.

Kamen's black box first connects to an electricity source. Next, you hook it up to a water source by dropping the hose in some water. The dirty water gets sucked into the system, where it warms to its boiling point (212 degrees Fahrenheit or 100 degrees Celsius). Then, it enters an evaporator, where it's heated a little more and boils [source: Pacella]. Already, some contaminants are lost. Anything that boils at hotter than 212 degrees F (100 degrees C) -- stones, dirt, salt -- stays in the evaporator and is drained out. Bacteria, viruses, eggs and spores get hit twice: They don't rise with the steam in the evaporator and are pasteurized by the heat in the purifier.

Steam rises from the evaporator into a compressor. The compressor squeezes the steam a little, raising its temperature a bit above 212 degrees F. The steam flows into an outer chamber whose walls are about 212 degrees F, creating another filtering step [source: Pacella]. Any contaminant that boils at colder than 212 degrees F, such as benzene, remains a gas and is vented out. Only pure water condenses on the walls.

The clean water drips into a final chamber, ready to be spouted out. But there's a problem: The water is still hot. Since hot water would be awful on a hot day, the machine cools it using a clever method. It flows incoming and outgoing streams of water past one another, so dirty water heats to 212 degrees F and outgoing water cools to the outside temperature. This heat recycling trick is called a counter-flow heat exchanger.

While the Slingshot is a powerful purifier, there's a catch. Read on to find out what it is.