What it shows:
A supersaturated solution is unstable, and by seeding it you can trigger rapid crystallization.
How it works:
Sodium acetate can dissolve in water in great quantities at high temperature, and if you let the solution cool carefully to around room temperature, you have a clear supersaturated solution. Disturbing this unstable equilibrium by dropping a small crystal of sodium acetate into the solution makes the whole thing solidify; the sodium acetate crystals growing radially outwards from the impact point of the seeding crystal.
Setting it up:
Here's the recipe for your supersaturated solution.
1. Place 50g of sodium acetate trihydrate in a clean 200ml flask.
2. Add 5ml distilled water and slowly warm the flask.
3. Swirl the flask until the solid dissolves completely. Make sure that none of the solid remains in the beaker (use a small quantity of distilled water if need be).
4. Remove the flask from heat, wrap it in aluminum foil and allow to cool to room temperature (4+ hours).
Prepare three or so flasks as insurance against accidental triggering (we've been lucky so far, so we're not sure how much punishment they can take before they solidify). Mount a camera above the flask, 10mm wide angle lens, with dark background beneath. You'll need a pair of tweezers and several sodium acetate crystals as seeds.
An experiment that takes under a second from start to finish. The surface patterns are beautiful, with spiky crystals radiating out from the impact point and even the impact ripples frozen solid. It is however a better demo for the demonstrator than for the audience.