What is this magic, which allows heat to be made in a small packet, without flame or electricity? Surely it is made by wizards! Ah, but no. It's science!
Handwarmers work their magic through the power of rust. Yes, the same thing that destroys your car in the winter heats your hands up as you drive. The ingredient that puts the oomph in heat packs is iron. Small pieces of iron are dispersed in heat packs, isolated by the wrapping on the pack from any oxygen. When the wrapping comes off, the permeable membrane of the pack lets oxygen in and makes it "oxidize" the iron. Oxidation is what causes a lot of the heat on earth. When a tree burns down, the carbon in the tree is oxidizing, too. Both iron and carbon give off heat when they oxidize.
And yet the iron around us isn't constantly sizzling. The iron in the heat packs is surrounded by a chemical that allows the reaction to go much faster. It's ordinary salt, and it's why a lot of important iron things get eaten through in the winter, when salt is regularly thrown on streets and sideways to de-ice them, and is then kicked up on to cars and buildings, getting a good rust started on them unless they're well protected.
Also in the heat pack carbon, which spread the heat around, and vermiculite, which keeps the heat from escaping too fast. The slushy feel of the pack is due to water and polypropylene, a polymer with high heat resistance that keeps the water from evaporating.
Eventually, all the iron oxidizes and the heat dissipates. But the rust... the rust will last forever.
[Via Bring on the Heat]