How reusable heat packs work
by Mark Frauenfelder from Boing Boing on (#77S59)
I took a home blood test today, and it included a reusable heat pack. These things have been around since the 1970s, but this was the first time I've used one. It's a plastic pouch with liquid inside and a small metal disc stamped with many dents. - Read the rest
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