In the 1890s, the German chemist Wilhelm Ostwald recognized that supersaturated solutions often preferred to transform into metastable intermediate phase rather than thermodynamically stable one ...

More information: Yong Chan Cho et al, Impact of molecular symmetry on crystallization pathways in highly supersaturated KH 2 ...

The second is the supersaturated solution (aka crystallisation-type) hand warmer, which involves a little more work on your part. You immerse this in hot water until it has absorbed the heat.

Commercial heat packs (containing iron and water, or supersaturated sodium acetate) and cold packs (various ammonium salts) can be used to show exo- and endothermicity. Heat packs that contain iron ...