In the southwestern United States, where years of drought are leading water managers to consider drastic water-provision measures such as desalination and cloud seeding, entrepreneurs have suggested reviving a water-saving technique that was tried and abandoned half a century ago. They propose to stretch dwindling water supplies by slowing down evaporation from reservoirs by means of a surface barrier of cheap, non-toxic, biodegradable chemicals just one molecule thick — two-millionths of a millimetre.