Millions of black plastic balls are helping make California’s drinking water safer and reduce losses from evaporation, The New York Times reports. Known as “shade balls,” the floating orbs are said to block sunlight and ultraviolet rays. Blocking those rays can potentially have a twofold benefit for water quality. First, it prevents light from reacting with bromide and chlorine in the water to form bromate—a suspected human carcinogen—and it can help prevent algae from growing, officials say. Plus, the balls are expected to save (a bit) of water, preventing the loss of about 300 million gallons annually by slowing the rate the evaporation from the Los Angeles Reservoir where they were dumped. The reservoir holds 3.3 billion gallons of water, or about a 3-week supply for the city.