Where does this mysterious heat come from? A chemical reaction occurs when the air mingles with the iron. This produces heat so it is called an exothermic reaction.
All of the ingredients in this "experiment" play a role. The iron is the energy source. The chemical reaction is boosted by the salt and the charcoal like gas powers a car. Water serves as the combining medium, which is evenly diffused by the vermiculite. Lastly, the pencil shavings insulate the hand warmer so that it retains heat..