Question

Why new earthen pots keep water more cool than old earthen pot?

Answer 1

Their are fine pores in new earthen pots due to which water gets evaporated fastly. But In old earthen pots the pores that is capillary is filled with dust particles.,due to which evaporation not takes place and water not became more cooler compared to new earthen pots.
Hope you understand

Answer 2

Water looses its temperature through small pores present in the pot. Hence in an earthen pot (matka) become cool during summer.