Why does foil touching a tooth filling taste strange?

Chris - Did you ever do the thing when you were at school where you put your metal pencil sharpener in your mouth?

Helen - I've never tried!

Chris - Certain pens that have a clip that you put over your pocket and it's made of metal, a different metal to the pen. You suck those and occasionally you notice a tingly sensation in your mouth. Have you ever noticed that? Helen - I'm going to try now but my pen's plastic.

Chris - That won't work. The point is that when you mix two different metals together and you have an electrical conductor between them and you have an electrolyte - saliva has lots of salt in it so it's an excellent conductor. You can get a chemical reaction happening between the two metals. One metal, the more reactive on will form ions and it will give up electrons which will flow through the electrolytes to the less reactive metals. That's how ht e reaction occurs. As a result if you tough two metals, in this case you're touching with a filling - that's mercury-silver amalgam - the aluminium will dissolve to make some aluminium ions. It'll make some electrical current which you will experience in your mouth as this tingling sensation and it will also react with the saliva to produce some hydrogen and some oxygen gas perhaps on the two different surfaces. You will actually deposit some material on your filling. You're basically turning your mouth into a battery.