How does the water kept in an earthen pot (matka) become cool during summer?
An earthen pot has a large number of tiny pores on its body.
Hence, a small amount of water always keeps seeping out from these pores and gets evaporated.
The water takes the latent heat of vaporization from the water inside the pot for evaporating.
Hence, the water inside the earthen pot keeps losing heat, thus remaining cool.
Thus, water kept in an earthen pot (matka) becomes cool during summer.