Have you ever had a drink of cool refreshing water from a matka or earthen pot placed outside? Surprisingly enough, the pots are exposed to blazing sunlight, yet the water within stays so cool. How is that possible?
This is because of a physical process known as evaporation. When a liquid changes to a gaseous (or vapour) state without boiling, it is known as evaporation.
A matka is made of mud and has many minute pores (extremely small holes). No matter how tightly you pack the mud, these pores remain. 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 to this process of continuous absorption of heat from the water inside the matka, in a few hours, this water becomes cool.
Glass and metal do not have any pores. So the water placed inside glass and metal vessels cannot seep out and evaporate. Therefore, the water inside remains hot.
Evaporation: nature's air-conditioner
In nature, there are countless examples of evaporation. For instance, have you ever noticed how a dog hangs its tongue out after running around? The saliva on its tongue evaporates and the tongue becomes cooler. In turn, the dog also feels cooler.
Why do we feel cool under a fan? This is because the sweat, that is oozing out of the pores in our skin, evaporates in the fan's breeze. The heat needed for the sweat to evaporate is absorbed from the object it comes in contact with (our body) and so we feel cooler.
You know, an adult marching through a desert may sweat up to 10 litres of water a day! A person may lose a lot of body salt through the sweat if he perspires too much. This can result in painful cramps, especially if he drinks water without adding salt to it.
That is why you must remember to drink lots and lots of water before going out on a hot afternoon.