Going to have to post a drawing soon, but until then a description will have to do. Imagine a dome made of a clear semi-flexible plastic. Now curl the bottom edges inside to create an inner trough. If you floated this plastic hemi-bubble on a body of water out in the sun, evaporation and condensation would sooner or later fill the trough with pure water. Could a device like this be low-tech enough to create cheap fresh water in underdeveloped countries? Consider versions of various sizes. Imagine one a few feet across, but put a drawstring along the inside rim, so that when the trough is full, a yank on the string turns the bubble into a bag that a person or animal could conveniently carry. (Such bags might be placed on wastewater ponds, manmade canals, or perhaps even the ground.) At the other extreme, huge hemi-bubbles could be dropped off at sea, so they can turn seawater into fresh water. Perhaps, after unloading oil, tankers could pick up Peter’s Pure Water Bags (got a good name?), or pump out the contents, to transport back to oil-rich but arid places lacking enough fresh water. Seems so simple, I suspect this is another idea others have thought of. If not, steal my idea, please, for the benefit of others.