system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does those heat packs with a tiny metal disc inside work?
gpt
The packs contain a liquid (actually a gel, usually made from a solution of sodium acetate) which has the property that it releases heat when crystals form in it.

When you flex the disc it simply causes a sudden sharp point to occur within the substance. This is enough to start the crystallisation process which then rapidly spreads throughout the entire gel. It therefore heats itself up and then gradually emits that heat over the next hour or so as it slowly cools down.