What is the heating principle of a baby warmer (hand warmer)? What is loaded inside?

Warm baby inside is composed of raw material layer, gelatin layer, non-woven bag of three parts, raw material layer placed in a non-woven bag, gelatin layer used to make the accompanying warm adherence to the clothes. The raw material layer is a polymer synthesized from iron, stone, activated carbon, inorganic salts, water, etc., which can undergo an exothermic reaction under the action of oxygen in the air.