system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is it when a tooth filling comes in contact with tin foil (that stuff you cook with) it hurts so bad?
gpt
You create a small battery and give a small shock to your tooth.  The aluminum foil is one electrode, your filling is the other, and your saliva is the electrolyte...the solution that allows electrons to move from one electrode to the other.