sphenopalatine ganglioneuralgia - n., also known as "brain freeze", occurs when the temperature of your palate doesn't have time to normalize between spoonfuls of cold substances, causing the capillaries in the roof of your mouth to dilate rapidly, filling with warmth-inducing blood. The sudden expansion can trigger nerves at the back of your palate to fire off urgent messages to your brain, like "Oh my god, the head is freezing!" The blood vessels in your forehead then swell in a patter similar to a migraine (but lasting less than a minute).
To cure brain freeze, according to Mark W. Green, director of headache medicine at Columbia University, press your tongue hard against the palate to thaw it quickly or breathe into your hands to heat your mouth.
[ via Ada Brunstein, WIRED magazine, June 2008 ]