Question

how does the water kept in an earthern pot become cool during summer

Answer 1

because in the pot evaporation takes place and we know that evaporation leads to cooling

Answer 2

Earthen Pots are made of porous clay, which slowly makes the inner surface of the pot wet and when water inside the surface move to the outer surface of the pot it evaporates taking the heat (form of energy) from the surrounding and from the surface of the pot, thus bringing temperature down and making water cool inside.