"Lobster" wrote in message
...
My daughter was given a 'handwarmer' for Christmas, comprising a plastic
sachet about 3" square and filled with some form of gel. Also side is a
small bit of bent metal; you bend the metal piece so it 'clicks' and bends
in the opposite direction, which initiates a reaction causing the gel to
solidify over a period of several minutes, giving out substantial heat.
Later you bung the thing in a pan of boiling water for 20 mins to melt the
solid back to a gel and reverse the reaction. Not exactly environmently
friendly use of energy, but hey...

So she asked me how it worked.

"Er...."

I vaguely remember stuff from my schoolboy chemistry about seeding
crystals and latent heats and so on, but I'm lost. Anybody care to
explain it for her?!

Thanks
David

You would be better looking at Google for a site called "howstuffworks" -
I'm sure you're daughter would have been able to tell you that!