Brain Freeze is getting a suddenly high-pitched ache in the upper part of the throat (Nearly in the head) because of consumming too quickly a cold drink or a cold food. The pain stays for about 30 seconds, but it's intense cold makes you feel like your brain is going into hypothermia and die.
Therefore, there is no need for Aspirin or calling 911 as it will go away soon.
Henry: Let's have a Slush drinking contest!
Bob: Alright! 3, 2, 1, DRINK!
*Both drinks like they were in need of weed*
(10 seconds later)
Henry: URRGH! MY HEAD!!!!!
Bob: Brain Freeze!!