"This works because the substance is not "ice" as in water, but a different substance called sodium acetate trihydrate. Normally solid sodium acetate was melted into a liquid that was then supercooled to below its melting point. In this state, adding nucleation sites, like the crystals on my hand, rapidly causes the rest of the sodium acetate to crystallize."
Aaaand saving for future reference.
As all species in the genus, Stapelianthus arenarius (Gentianales - Apocynaceae) is a stapeliad native to Madagascar, distinctive by its white flowers with five fleshy sepals (occasionally four), spotted red-brown in the center.
Photo credit: ©Mike Keeling | Locality: cultivated (2009)