This kettle will heat 16 oz of water in full sun to a nice hot/warm temperature for some sun tea or washing hands, but you'll have to wait a few hours or less depending on outside temperatures. I've only recently made this during the winter and have achieved 95 degree water temps during 30 degree outside temps. Someone in a warm area of the world will have to make one and tell me how hot it can get during summer-like temperatures.
(Instructables member Kopomeroy built a few versions of this; they achieve 140-160 F during 80 F outside temps. See comments below.)
This takes three plastic bottles to build, one 2-liter, one 16 oz, and one 20 oz. Also needed are flat black spray paint, 5/8" (16mm) rubber heater hose, aluminum foil tape. The outer 2-liter bottle should preferably be clear.
After making this I realized a major problem when using a plastic bottle core; the breakdown of the plastic from heat and UV rays which could leach plastic chemicals into the water. This hot water might be useful just to wash your face or something but not to drink. An alternative to this design is to use a glass or aluminum bottle for the center bottle.
Step 1: Clean, prep, and paint
Peal off all the labels.
Give the 16-oz bottle a light spray with the flat black so it drys fast, a second coat may or may not be necessary.