I thought this was a pretty cool idea. Basically just dig a hole, put a cup in center, then drape plastic bag over it with a rock in the center. What happens is the sun evaporates the water in the ground (or maybe water that was there) and it condenses against the plastic and drips in the cup due to the angle of the plastic from the rock on top of it. No purifying necessary even for chemicals. Here is a decent and explanation/visual http://en.wikipedia.org/wiki/Solar_still. also all you really need is just a plastic garbage bag and some rocks laying around.