How does the water kept in an earthen pot (matka) become cool during summer?
Evaporation is a process when a liquid molecule loses its energy and converts to a gaseous molecule. This is what happens to water inside the pot.
Pot surface made of earthen clay have a large number of pores over their surface. The liquid molecule which will be having the highest energy level among the pot water will convert into a gaseous molecule first. So what will be left behind in the pot water, will be the molecules of water having lesser energy, the cooler water molecules. If this process keeps going on for hours, subsequently the water in the pot will become cool.