1) evaporation rate is high when surface area is more.

2) dogs have toungues with large surface area.

3)so, when dogs pant evaporation takes place it provides them cooling effect.

4)evaporation is a cooling process.

(evaporation:the process of escaping of molecules from the surface of liquid at any temperature is a evaporation.)                                                                               
* the dogs only a place where a dog sweat is on its foot pads and rest of its body is covered in fur coat that it can't take off.since it can't sweat
* that why dogs pant to cool.                                          
* when a dog open his mouth and pants,its releasing moisture the best way it can.
 *so,when it open up its mouth,his tongue actually expands and dog pushes heat straight out the body*
 (evaporation is a surface phenomenon and cooling process)