An earthen pot is made of mud and becomes porous with a very fine holes all over. When water is poured in the pot, it first spreads itself on to the walls of the pot.This water percolates in the pores to the outer surface. Here evaporation of water takes place thereby producing a cooling effect. The pot becomes cool and also makes the eater inside cool. If you observe, the cool water level is lesser than the intial water level when poured.