This question was asked by Sarah from Chichester, UK.
Brain freeze, or to give it its proper name sphenopalatine ganglioneuralgia (I think we’ll stick with brain freeze!), is the sudden headache you can get when eating or drinking something very cold. Your mouth and tongue contain a lot of blood vessels, which is why we commonly use thermometers there to check the body’s temperature. When the roof of the mouth (the palate) gets chilled from sudden or prolonged contact with something cold, these blood vessels dilate in an attempt to get more warm blood to the cold area.
Because the palate is so close to the brain this also causes an increase in the blood flow through the anterior cerebral artery. This increase triggers pain receptors which send signals along the trigeminal nerve to the brain to warn that there’s a problem (brain freeze is also sometimes called a trigeminal headache). Because the trigeminal nerve is also responsible for sensing pain in the facial area, the brain interprets the signal as coming from the forehead, which is why you feel the pain there. This is known as referred pain.
Brain freeze typically occurs about 10 seconds after the palate is chilled and lasts for 30 seconds or so. There’s no definite way to prevent brain freeze but eating or drinking cold substances slowly makes it less likely to occur. If you do get it, pressing your tongue to the roof of your mouth or sipping a warm drink will cause the blood vessels to constrict and hopefully make the brain freeze end faster!