Ah, the art of distillation from humble materials is as ancient as the desert itself. Place the tin inside the larger tin, leave a gap, fill the gap with a small bowl of saltwater. Cover the top tin with the plastic bag, seal the edges, and place the bag over the smaller tin so that its lowest point is in the water. Sunlight will evaporate the water, condense on the bag, and drip into the bowl. The key is to keep the system as airtight as possible and to ensure the bag's lowest point stays below the water level to collect the condensate efficiently. Remember, patience is the best lens in this experiment.