In this video demonstrator stick his hand into a jar full of molten sodium acetate and have it crystallize with his hand inside.

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.