During hot summer days, water is usually kept in an earthen pot (called pitcher or matka) to keep it cool. The earthen pot has large number of extremely small pores (or holes) in its walls. Some of the water continuously keeps seeping through these pores to the outside of the pot. This water evaporates (changes into vapor) continuously and takes the latent heat required for vaporization from the earthen pot and the remaining water. In this way, the remaining water loses heat and gets cooled.