In the old days, a clay pot cooler (also called a zeer or pot-in-pot cooler) was often used for keeping foodstuffs fresh in very hot and dry areas. Water was used to cool the inner pot as it flowed through a porous surface (such as sand) and subsequently evaporated. What is the minimum temperature that can be achieved using such a device and what does this minimum temperature depend upon? Explain what determines the rate of cooling?