Crystallization of sodium acetate - GIF

[hemmjo]

I recall an activity from a LONG ago physics class where we melted some white solid material, I do not remember what it was, then it cooled but did not solidify. When we touched it with a toothpick and it hardened almost instantly.

Is that what is going on here? or is that a saturated solution waiting for someplace for the crystals to form?