Solar Water Purifier from Discarded Plastic Bottles

Our school is in a village where the groundwater is salty for part of the year. During a lesson on evaporation and condensation, a student asked whether we could "make rain inside a box". That question became a six-week project. The students collected 2-litre plastic bottles, cut and joined them to form a sloped condensation chamber, and painted a tin tray black to absorb heat. Salty water is poured into the tray, the sun evaporates it, and clean water collects along the inner wall of the bottles and drips into a cup. What they learnt: the water cycle, specific heat, angles of incidence, and a lot about iteration. The first version leaked at every joint. The fourth version produces about 1.5 litres on a sunny day. Cost of the whole unit: under ₹120. We have now built six of them and installed two at the Anganwadi next door.