A tiny, black rectangular device could be the world’s long-term solution for clean water.  The gadget is solar-powered, half the size of a postage stamp, and it disinfects water. “Our device looks like a little rectangle of black glass,” said Chong Liu, lead author of a paper on the device published in Nature Nanotechnology. “We just dropped it into the water and put everything under the sun, and the sun did all the work,” she said.

With 663 million people in the world not having access to clean water, a device this efficient stands to significantly change many lives. Roughly 800 children die a day due to lack of access to clean water.

The purifier, developed by researchers at the SLAC National Accelerator and Stanford University, contains microscopic layers of “nanoflakes.” When exposed to water and sunlight, they produce hydrogen peroxide, a natural disinfectant. In early testing, the disinfectant killed 99.999% of bacteria before dissipating and leaving behind safe-to-drink water. “When you see there’s no bacteria growing, it’s really exciting,” said Liu in the report published on Monday. “We didn’t expect it to work that well at first.”

The new device is a substantial improvement over previous solar-powered water cleaning systems. The nanoflakes, which contain industrial lubricant molybdenum disulfide, absorb more than just UV light. Absorbing a wider band of the light spectrum means the nanoflakes utilize 50% of the incoming sunlight’s energy, whereas standard purifiers harness 4%.

Read more: This Tiny Device Could Provide Clean Water for 663 Million People