A fun thing to make. I made this in the high school chemistry room after school. Filter paper with some iodine crystals, pour some ammonia over and wait for it to dry (as I recall). I am not sure where I learned about it. Often my dad, who was a chemist then, would tell me little tricks like this. (He also turn me on to slathering a small amount of potassium permanganate with glycerin.) Anyway, I was walking with it…
He also liked to let off expired fire extinguishers at his sixth formers out of his window, and he once attempted to use them to propel a pupil sitting on a wheely office chair. Very memorable teacher!