Cats gets brain freeze. Why?
Why do cats get brain freeze? Well, in this compilation on YouTube they get what is called "brain freeze" because they are eating ice cream. The reason why they like eating ice cream is because it contains quite a lot of fat in the milk and cream. Cats always like that. But then they find it is cold which causes pain and discomfort so they stop and open their mouths and pause giving the impression of 'brain freeze'.
I can only presume that they do this for the reason that (1) it allows them to warm up the interior of their mouths quite quickly because the ambient, warmer air impinges upon it and because (2) they are in pain and pull a face as humans would under these circumstances. When it is warmed up they continue to eat ice cream. It's as simple as that in my view.
Wikipedia provides me with a more technical description of feline brain freeze. They say that it is a cold-stimulus headache which is a brief form of headache associated with the consumption of cold food or beverages such as ice cream.
The authors argue that the coldness of the food in touching the roof of the mouth results in a nerve response which in turn causes a rapid constriction of blood vessels. The capillaries in the sinuses constrict and then they warm up and dilate. This is picked up by pain receptors which send a signal back to the brain. It is referred to as "referred pain".
Brain freeze pain may last for a few seconds to a few minutes. They do not explain why the mouth is held wide open which common sense dictates is for the reason that I have stated.