Heat-pads that Click

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A kitschy "Crafts Exhibition" in Thiruvananthapuram yielded something intriguing - an unusual heat-pad.

Packaged as green, viscous liquid in clear plastic pouches, each pouch has a coin-shaped piece of metal, which, at first glance, looks like something that got sealed-in by mistake. It is, in fact, the "switch". Click this with your fingers and a chemical reaction is triggered, rapidly turning the whole pack into a hot, crystalline colloid!

What is inside the packet? How does it work?

The salesmen had no idea. Their mandate was only to parrot a sales-pitch and to ensure that each packet got sold for no less than Rs.250 ("..or Rs.499 if you buy it from a drugstore").

So, once again, we have WWW to the rescue. According to one of the respondents at PistonHeads, the pouches contain Sodium Acetate Trihydrate:

Sodium acetate trihydrate crystals have a melting point of 58 °C. When they are heated to around 100 °C, and subsequently allowed to cool, the aqueous solution becomes supersaturated. This solution is capable of supercooling to room temperature without forming crystals. By clicking on a metal disc in the heating pad, a nucleation center is formed which causes the solution to crystallize into solid sodium acetate trihydrate again. The bond-forming process of crystallization is exothermic, hence heat is emitted.

The answer is presented in more detail (with a video) at HowStuffWorks. Sodium acetate is also called "hot ice" and is food additive, commonly used in "salt and vinegar" flavored chips.

The heat-pads make excellent gifts - especially for the elderly.