Biting on tinfoil

I recently had a temporary metal crown (now replaced with a permanent, ceramic one); and while the temp was in place I occasionally would hit it accidentally with a fork. This would cause a sensation familiar to anyone who has ever done the act in the OP: not exactly painful, not really a taste, sort of like an electric shock – unpleasant, and weird.

What exactly is going on in your mouth, when metal touches metal?

You are creating a little battery. The dissimilar metals create the anode and cathode of the battery, and your saliva is the electrolyte. This causes a small current to flow, and your nerves are in pretty much direct contact, so it doesn’t take much current to feel it.

Fascinating. Thanks, beowulff. Makes me wish my fork had a lightbulb or a propeller on the end; I could do tricks.

Well, it worked for uncle Fester…

There is such a thing. You can sometimes find it in novelty shops. It looks like an ordinary incandescent light bulb. Stick the base in your mouth, and it lights up.

Look up Uncle Fester.

Details of this phenomenon and OP’s subsequent query are discussed in If I had a lightbulb in my mouth and a lightning bolt hit me, would it incandesce?