2. One brand of reusable physiotherapy heat pad contains a supersaturated solution of sodium ethanoate and a metal disc. Sodium ethanoate is non-toxic and can be used as food additive. The solution crystallizes when the metal disc inside the pad is bended. The heat of crystallization is substantial and produces a moderate amount of heat. The temperature of the pad may rise to above 50° C as crystallization occurs.

3. 1.Boil water in a pot. 2.Place a cloth on bottom of pot. 3.Place the pad on cloth inside the pot, and continue to boil water until all crystals are dissolved, and the pad is completely clear.

4. 4. When the pad is completely clear, take it out of the water and place it on a cloth to cool down. If even one tiny little crystal is left, the liquid will quickly re-solidify. Hold the pad up against a light and make sure there are no crystals left. If there is even one tiny little crystal, continue to boil until it melts. 5. When the pad cools down, it is ready to be used again. 6. If crystals start forming while the pad is cooling, repeat steps 1, 2 and 3.

5. http://www.howstuffworks.com/question290.htm http://www.nahpco.com/heat_packs_small.htm Video clip