system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 - Why does biting aluminum foil hurt?
gpt
Because you probably have metal fillings in your teeth.

Take two dissimilar metals and a conductive medium between them, and (depending on the metals), current will flow between them, carrying ions away from one surface and plating them onto the other. 

This is the operating principle behind batteries. They're effectively lumps of two different metals with some salty water between them. 

Since your fillings aren't aluminium, biting down on the stuff makes a mini battery in your mouth, electrocuting your teeth just enough to be unpleasant.