1. materials: boxes (preferably iron boxes), small cans (must be made of metal), foam, reflector (tin foil, mirrors, waste CDs in cigarette cases can be used ...), adhesive tape, black pigment and cellophane.

2. The boxes and small cans should be painted black (so that more heat can be absorbed). Dig a hole with foam (also painted black) so that it can be vertically stuffed into 3/5 of the small cans (for thermal insulation), and 2/5 can be exposed to receive sunlight. Put the foam even into the box, seal it with cellophane, and make a support behind the box, so that the box can stand obliquely against the sun. (The bigger the reflector, the better), the reflected light should be able to shine on the box or small pot, so that the box and small pot can absorb more heat. A simple solar water heater is made.