Kids, let's explain why chewing an aluminum foil spitball can really hurt some people, while for others it is just a weird piece of gum. 

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

The difference is because some of us have silver fillings in our teeth. It turns out that aluminum atoms lose their electrons very easily. In the presence of our mildly acidic saliva, which acts as a catalyst, we have what amounts to a crude electric battery. Electrons flow from the aluminum to the silver amalgam filling. The current is felt by the nerves of our teeth and causes a downright unpleasant zing!

Aluminum foil will begin to decompose in the presence of many other acidic substances in a process called oxidation. Acids like to oxidize obliging metals. Some common acidic foods include ketchup which has a pH of 3.8 (7 is neutral), or a cola soda which is even more acidic with a pH of 2.7. Tell any cooks you know to never wrap a meatloaf glazed with ketchup or tomato sauce in aluminum foil for storage. After several hours the result of this contact is a grayish-black disgusting mush of aluminum oxide.

Brainteaser: why doesn't a full aluminum cola can dissolve? Chemistry solves that problem, too. The inside of the can is coated with a harmless but effective protective surface made up of long molecules called polymers (in short, a plastic coating).

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
October 1993

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Reference: (from p. 121 of The Straight Dope by Cecil Adams, 1984).