Why do we get "brain freezes" when we eat cold things too quickly? what is the underlying definition of a brain freeze?

Brain freeze?: When something super chilly hits the roof of your mouth, the nerves there zing a message north that makes your blood vessels spasm. That, in turn, causes your headache, aka. "brain freeze." i scream! you scream!

Brain freezes: When you eat or drink something cold, the cold thing touches the roof of your mouth, which causes the blood vessels to constrict in an attempt to prevent loss of body temperature. As the coldness fades, the blood vessels dilate, quickly increasing blow flow to the brain. This sudden increase of blood flow in the brain causes intense headache as you describe. Eat cold things slowly!

SCUBA: Interestingly enough, scuba divers in cold water who don't protect their forehead get exactly the same sort of headache for exactly the same reason. The sudden cold causes vasoconstriction to save heat, which causes increased flow to keep the brain going, causing the headache. Stay warm!