Hot pads
When our kids were little, we had warming pads that were plastic bladders filled with some kind of liquid (not water.) Inside the bag was a small, metal disk like one of those "cricket" noise makers. Pinch the disk, and it goes "click." The whole bag suddenly crystallizes, and gets hot. Instant hot pad. It could be "recycled" by boiling it and carefully letting it cool undisturbed.
In the case of this material, the reaction was predictable enough to be made into a saleable product.