In this video a man sticks his hand into a jar full of molten sodium acetate and has 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 the man’s hand, rapidly causes the rest of the sodium acetate to crystallize.