Why does metal cause pain when touching a metal tooth filling?

Dave - One of the things that could certainly be happening when you touch two metals together in your mouth, where there's a load of electrolytes, is you've essentially made a battery. If you have two metals of different reactivities, the most reactive one will tend to form ions. So if you've got aluminium foil, that's quite a reactive metal. That will form Al3+ ions that will dissolve in your saliva, which acts as the electrolyte. In the process, the less reactive metal will have a load of extra electrons given to it and it will have to somehow get rid of those electrons. So, if there are any dissolved things nearby, salt metals for example, they will deposit on the surface. Alternatively, there could be some other electrochemistry going on. My guess as to what's going on in your filling is that some of that electrochemistry is happening near the nerve in your tooth and it's annoying the nerve and causing you some pain. Chris - [The pain could result from] passing some of the electrons that result from the battery you've made in your tooth filling into the nerve and directly stimulating it, or perhaps it's making some gas locally by reducing some hydrogen ions in the solution. That would make some hydrogen gas which increases the pressure inside the tooth, and that could put pressure on the nerve and make you feel like you've got toothache, which is caused by pressure inside the tooth because of the activity of bacteria irritating in there. Dave - Or even just producing something slightly poisonous, which directly chemically annoys the nerve.