Here’s how it works.

You take two earthen pots, both being the same shape but different sizes, and put one within the other. Then, fill the space between the two pots with sand before pouring water into the same cavity to make the sand wet. Then, place food items into the inner pot, and cover with a lid or damp cloth. You only need to ensure the pot-in-pot refrigerator is kept in a dry, well-ventilated space; the laws of thermodynamics does the rest. As the moisture in the sand evaporates, it draws heat away from the inner pot, cooling its contents. The only maintenance required is the addition of more water, around twice a day.