Brain freeze is so serious it has a scientific name: sphenopalatine ganglioneuralgia. You drink or eat something cold very fast and BOOM, your head feels like someone's trying to blow it up with their psychic powers.
So, why the heck do we get those awful brain freezes? Well, your mouth is highly vascularized, meaning there's a lot of important blood flow going through there. It makes sense since it's right there between your head and neck. Plus, two super important arteries are right in the back of your throat: the internal carotid artery, which is what gets blood to your brain, and the anterior cerebral artery, which sits right where your brain tissue starts.
So right there, in the back of your throat, is where your brain's getting all the stuff that's keeping it alive. And it's a pretty soft, vulnerable part of you.
When you eat something cold really quickly, all of a sudden the temperature in that part of your body drops. FAST. And those arteries contract really quickly, triggering receptors in your meninges, which is your brain's outer covering.
Your brain is basically alerting you to the fact that it thinks you're suddenly freezing it to death, even though it's just one spot on your body that's cold.
Neuroscientists actually really love to study brain freeze because it's the only controllable headache we have. They can't make someone have a migraine, but they can give them brain freeze then study what's happening in their brain and body and use that to treat other, more serious types of headaches.
By the way, if you've got brain freeze, the best thing you can do to get rid of it is to curl your tongue up into the back roof part of your mouth- that's that spongey bit that's the border to the arteries.
Your tongue will still be body temperature and you can use it to equal things out quickly.