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Mystery #2Tier 2-3chemistry / energy

How does a hand-warmer get hot without any fire?

On a freezing day you snap or shake a little pouch, slip it in your glove, and minutes later your fingers are toasty. There's no flame, no battery, no plug. So what's making all that heat appear out of a small soft packet?

Petri dishes of sodium acetate crystallising into radiating needle patterns, the chemistry inside a reusable hand-warmer.
Photo: Milos Aleksandric, Wikimedia Commons · CC BY-SA 4.0 · via Wikimedia Commons

Your guess

What's your hunch about where the heat is coming from if there's no fire and no battery?

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