You folks reminded me of another. Certain high end plant vendors ship their plants (such as orchids) with a crystal lattice heat generating pad, Geeze, sorry I don't know what they are called. Maybe just "hand warmers?" Anyway they are little pouches that have a crystallized matrix in them. You break the crystals and the heat of the crystal lattice is released, melting more of the crystal, thereby releasing even more heat.
The plant vendors use them to keep delicate plants from freezing in shipping. But they are re-usable. Just take the pad, pop it into a pot of boiling water. Let it cool. As it cools, the crystals will re-form and you can slip it into your pocket for when you have to walk the kids to school or catch a bus or whatever.
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