(YouTube Link)
The Pharaoh's Serpent is a traditional pyrotechnic and chemical display in which mercury thiocyanate is set on fire, producing what looks like a very rapidly growing vine. It has since fallen out of favor due to the toxicity of mercury, but sodium bicarbonate produces a similar effect. The video above is a demonstration of this visually stunning chemical reaction.
via Urlesque
http://chemistry.about.com/od/chemistryhowtoguide/a/blacksnakes.htm
We used to make a big pile of the old ones and light them all at once. With enough tablets, you could crack a sidewalk. God knows what the fumes were doing to us.
The resulting stuff would make a great sci-fi background for a stop-motion film or something.