Why dog keep its tongue outside in very hot day ?

It is very hot in:

Why do we keep phosphorus in water?

It is difficult for most plants to carryout photosynthesis in very hot, dry environments why ?

Why does a desert cooler cool better on a hot dry day?

Why do pigs toil in the mud during hot summer ?

Why does it takes sometimes to see objects in a dim room when we enter the room from bright sunligth outside ?

Which region on the globe is very hot and which region is very cold?

What are responsibility for that the sky appears white sometimes in hot days?

A person in a dark room looking through a window can clearly see a person outside in the daylight, where as the person outside cannot see the person inside. Why ?