The demo was the instant ice effect of sodium acetate, where you pour a liquid onto a surface and it immediately solidifies into a phallic tower. I first saw this on QI last year, and meant to get around to trying it, and then I had one student in particular who was most insistent that I should try it, after their teacher last year apparently failed at it.
The reason it's supposed to work is that sodium acetate in solution has a much
Getting the balance just right seems to be key, because when I did finally get something like the right effect, my solution was still too 'wet' and the crystal tower was really just a crystal puddle. At the same time, as the liquid rushed out, too much sodium acetate got left behind, 'freezing' along the length of the test tube and down the neck of the funnel. That looked cool, but wasn't what I was aiming for. I'm not quite sure how to keep the solution more evenly concentrated.
Also, for cooling, I used a beaker of ice, with the test tube of solution stuck in it. That went ok, but couldn't have helped the even concentration, as the top half of the test tube was sticking out and presumably not cooling at the same rate. Silly mistake to watch out for next time.
One advantage I did have was starting out with a jar of acetic acid. This saved me all the horrible trouble of boiling off vinegar to get some.
I'd welcome any feedback on how to handle this better, if you can spot anything.