I used www.google.com and looked in "Groups" and found the answer in rec.pyrotechnics from a fellow named Chris Spurrell:

There's an old "Bar bet" wherein the person takes the Aluminum foil from a cigarette pack, wets is with some Hg2Cl2 and pinches it between both thumbs and forefingers, and says "I bet you can't hold in your hand what I've got in mine." The sucker accepts the "gift" of reactive Al foil and gets a burn as the air oxidation causes it to heat up. The Bichloride of Mercury used to be available as an antiseptic.

The chloride ion will break down the oxide layer on the aluminum, allowing the aluminum to oxidize assuming there's some water present to ionize the mercurous chloride.

It is called hypno heat and it is a magic trick using mercury compound. You can order it online. Search hypno heat and go from there. Some places only sell it to professional entertainers because of mercury involvement, and most of those places recommend not using it at all because of skin absorbtion and mercury poisoning possibilities.

You can use mercuric chloride (HgCl2) as well. It does the same trick. But both these compounds can be very dangerous if not handled carefully. I would not advise using anything that involves mercury. Both these mercury compounds easily combine with protein to form highly toxic organomercury compounds. If mercuric chloride absorbs through your skin it can cause problems with your nervous system and other things. Inhalation is another problem. From my own personal experience i can tell you not to try this trick. After watching our school instructor perform the tin foil trick, I took some mercuric chloride home and wanted to demonstrate to my friends. I accidentally got my fingers wet while rubbing the mercuric chloride onto the tin foil. To make a long story short, I ended up with some pretty serious chemical burns as well as heat burns. It caused some corrosion on my thumb, middle finger and pointer finger. Mercuric chloride is quite expensive, costing about $90 for 500 grams.