A bit hypothetical, but ruling out being able to use a solar still, or the SODIS method, could you make water drinkable by using the suns heat through a magnifying glass?
Paper self ignites at 451° F , water boils at 212° F, but that is at one concentrated point when paper burns using a magnifying glass. Would it be possible to boil a small about of water using the same method? For sake of it lets say you have a magnifying glass that is 3.5" across, and a plethora of different small containers available glass or metal.
I realize you could then start a fire, and boil water in a metal container but i am just curious if it is possible to heat the water enough to be drinkable without placing it on a fire using this method. As fires cause smoke lets say you are in a less than desirable place where the smoke from a fire could give away your location when you didn't want it to.