Sphenopalatine Ganglioneuralgia (Brain Freeze)
Having a kid around means fielding lots of questions, which can range from fun to horribly embarassing. When LRB (Lord Running Boy) ate his vanilla ice cream too fast, got hit with the dreaded brain freeze and asked, "how can ice cream make my head hurt?" Good question, my boy. Internet to the rescue.
The medical term is sphenopalatine ganglioneuralgia. When something very cold touches the top palate of the mouth, it causes the blood vessels to constrict. This makes the nerves send a signal to the brain to reopen them. The rapid reopening of the vessels causes buildup of fluid in the tissues causing a slight swelling in the forehead and, therefore, causing pain. It normally takes 30-60 seconds for the fluid to drain, and the pain goes away.