Hand warmers containing a liquid in a flexible plastic shell with a small, curved metal disc are commonplace. The warmer is activated by inverting the disc, which presents nuclei to support the formation of crystals from a supersaturated solution. Needles propagate out from the nucleation site and within a matter of seconds the contents have solidified. The process is exothermic, generating heat.
This effect can be reproduced in the lab to offer more control over the process.
A supersaturated solution of sodium ethanoate is poured onto a dish with a seed crystal of the solid sodium ethanoate on it. The liquid instantly solidifies and you can continue pouring to make a steaming ‘stalacite’.