A matka is made of mud and has many pores [extremely small holes] no matter how tightly you pack the mud, these pores remains. it is through these pores that the water placed inside the matka, oozes out. now to evaporate, the water needs to absorb heat which will change it to vapour. the only way the water oozing out of the matka can turn to vapour is by absorbing heat from the liquid within the matka and the matka itself. due this process continuous absorption of heat from the water inside the matka, in a few hours, water becomes cool.