furry_irl

Not your brain, the root of your tooth:

Biting on aluminum foil can be painful because it creates a voltaic battery in your mouth. This happens when the foil comes into contact with metal from dental work in a moist, saline environment like saliva. This contact generates an electrical current that flows to the tooth's root, stimulating nerve endings and causing pain, especially if you have fillings, crowns or other metal dental work.

https://science.howstuffworks.com/question564.htm