why is chewing on tin foil so painful?

I imagine it has to do with silvered fillings. I think the foil and the fillings react in an electrical manner and thus cause pain. I have the same thing happen to me, thus I have shunned all my tin chewing efforts.

I think its also a texture issue, it feels really foreign and disturbing

i've heard the same thing about silver fillings.. just to add some credit to that answer..

http://www.everything2.com/index.pl?node=fingernails%20where%20being%20dragged%20across%20the%20blackboards

Sorry, click on the "fingernails on the blackboard" category, there is a great explanation of icky tinfoil pain.

It's a reaction to the metal on metal with your silver/amalgam fillings which contain nickel.
Same thing can happen if you hit your metal spoon on a gold crown/or cap, on one of your teeth. Painful.

Because, in general, you’re chewing on metal (It’s made of aluminum). So, as any metal, It tears the tissue…

because you’re chewing on metal