Physics explains this principle well - when evaporation takes place, cooling occurs. This is because for evaporation to take place, the water needs to change into vapor or gas and this only happens when there is heat in the surroundings. So when the water absorbs heat, it evaporates and this makes the container or surroundings cooler. The earthen pitcher contains many pores or small holes. When water is poured into the pot, a small part of it exits through these pores and evaporates from the surface of the pot, thus making the pot (and remaining water) cooler than before.