I went for a run the other day when it was actually hot in Michigan. I got back to my apartment and grabbed a gatorade from the fridge. I started drinking and immediately got a brain freeze, but I was so thirsty I kept drinking. I'm finding that doing research puts me in a different mindset - the detective mindset. I started wondering what causes a brain freeze. My hypothesis I came up with was that it was from a countercurrent heat exchange between the cold gatorade going down my esophagus and the internal or common carotid artery. The thought was that the blood going to the brain cooled down as it passed the cold water in the esophagus and made the brain feel cold even from a slight temperature change. It sounds good.
However, the next day I was talking to one of the doctors about it and ended up googling it. The real reason is that nerves in the roof of the mouth sense the coldness and go to the brain. Yes, people have done research on this stuff. Apparently, a brain freeze can be avoided by pressing the tongue to the hard palate and heating it back up.