Due to galvanic shock
It is called Galvanic shocks. If you have a gray filling, which contains some metal, it can cause a shock when you put another piece of metal on it. This doesn't happen when you have white fillings. Another possibility is that you filling is due to be changed. The best thing would be to see your dentist to check it. · just now. Report Abuse. If you have a silver filling in your tooth and it touches something metalic like a fork or a piece of foil, it hurts because of a phenomenon called galvanic shock. The energy is conducted through one metal to the other and you get a shock. Pam · 8 years ago. Thumbs up. ­Biting on aluminum foil can be painful and is­ usually noticed if you have metal in your mouth from dental work (e.g. fillings, crowns).
http://www.answers.com/Q/Why_does_it_hurt_when_metal_touches_a_tooth_filling https://uk.answers.yahoo.com/question/index?qid=20070717113306AAz7Gue http://science.howstuffworks.com/question564.htm
why do fillings hurt when they touch metal