Hand warmers work by creating an exothermic chemical reaction that produces heat when the contents react with air or each other. Most commonly, hand warmers use pouches of fine iron filings that oxidize in the presence of air, water and salt, producing heat in the process.
Iron oxide hand warmers are cheap and easy to produce, but they only work once. Once the hand warmer is exposed to oxygen, the oxidation reaction begins and it cannot be stopped. The reaction continues until all the iron has reacted, at which point the heat dies down. Depending on the amount of iron present in the hand warmer, this may take several hours to complete.
Reusable hand warmers rely on the crystallization of a super-saturated solution to produce heat. These warmers usually contain a sodium acetate solution along with a metal strip. When the warmer is flexed, the strip bends and creates nucleation points for crystals to form. The super-saturated solution crystallizes, producing heat in the process. These warmers typically only last a brief time before the reaction peters out, but they can be recharged by boiling the hand warmer and melting the crystals back into solution.
A more dangerous form of hand warmer consists of a device that burns charcoal or lighter fluid inside an enclosed metal capsule. These were common during the early part of the 20th century, and could become dangerously hot if not managed carefully.