Question

dogs generally hangs out their tongue in summer. why?

Answer 1

They do this because in summer the feel hot and to make their body temperature cool down they hang their tongue.
Blood vessels present in it also cool down and hence carrying some cool blood to the whole body to maintain its temperature.
HOPE IT HELPS :-)

Answer 2

Dogs must hang their tongue out to keep cool in Summer. The evaporating saliva cools the blood that is passing through the tongue and mouth.