Serrador says no one really knows yet what causes them. But there are some theories. For example, Serrador has shown that just before the brain freeze hits, there’s an increase in blood flow to the front of the brain
“That’s increasing the volume and therefore increasing the localized pressure in that area,” he says. The brain may be interpreting that increased pressure as pain.
“Another theory that’s been put out there is that the cold actually stimulates a nerve in the roof of the mouth,” says Serrador. That stimulated nerve in the mouth goes into overdrive. It sends off a barrage of signals to the brain that once again the brain interprets as “ouch.”
Although why the brain gets ouch from the cold, and not brrrrr, is a mystery.