I have a couple of chemical hand warmers I use when out mountain biking. they are clear, flexible plastic 'bubbles' filled with a liquid. When the little metal 'widget' inside is flexed so it 'clicks' the liquid starts to turn into a solid and gets very hot. Simmering it in a pan of boilin water returns it to the liquid state.

What is the chemical inside the bubble and how does it work?

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