A charcoal cooler, also known as a zeer pot or pot-in-pot refrigerator, is a simple, low-cost refrigeration system used in many parts of the world where electricity is not readily available. The purpose of the charcoal in a charcoal cooler is to provide a cooling effect through evaporative cooling. When water is added to the sand or soil surrounding the pot, the charcoal absorbs the water and becomes moist. As the water evaporates from the surface of the charcoal, it takes heat with it, resulting in a cooling effect inside the cooler. The porous nature of the charcoal allows air to flow through it, which helps to speed up the evaporation process and enhance the cooling effect. By adding or removing water from the sand or soil surrounding the pot, the temperature inside the cooler can be regulated to keep food and other perishables fresh for longer periods of time.