1. Activation: You snap or shake the hand warmer to initiate a chemical reaction.

2. Sodium Acetate Crystallization: The reaction converts supercooled sodium acetate into solid sodium acetate crystals.

3. Heat Generation: The crystallization process releases heat, raising the temperature of the solution and the hand warmer.

4. Iron Catalyst: Iron or iron oxide helps speed up the crystallization reaction.

5. Exothermic Reaction: The process is exothermic, meaning heat is released.

6. Heating Duration: The heat lasts several hours as long as there's unreacted sodium acetate.

Reusable Hand Warmers:

1. Nucleation Point: These hand warmers have a metal disk that acts as a nucleation point for crystallization.

2. Bending: You bend or flex the metal disk to initiate the crystallization reaction.

3. Heat Release: The reaction generates heat, raising the temperature of the hand warmer.

4. Cooling: As the reaction completes, the hand warmer cools down.

5. Reboiling: You place the cooled hand warmer in boiling water to melt the sodium acetate.

6. Recrystallization: The sodium acetate recrystallizes when you remove it from the water and let it cool down.

Gel-Based Hand Warmers:

1. Click Activation: You click a metal disk or button to start a chemical reaction.

2. Exothermic Reaction: The chemical reaction creates heat through an oxidation process.

3. Gel Heating: The gel inside the hand warmer contains salts and water, which absorb the heat.

4. Heat Distribution: The gel evenly distributes the heat throughout the hand warmer.

5. Duration and Recharge: Some gel-based hand warmers can be recharged by boiling for future use.