The Leidenfrost Effect is a lovely sounding name for some very strange and nifty physics.
When you heat up a liquid, it will, eventually, boil away into a gas. Different liquids have different boiling points. But here's the weird catch: When you suddenly put a liquid in contact with something much, much hotter than its boiling point, the liquid doesn't instantly evaporate. Instead, it forms a little cushion of vapor between itself and the heat source. You can imagine it like a hovercraft moving over the surface of a lake. The cushion doesn't prevent evaporation—and it doesn't last long—but it does slow down evaporation enough that you can see the liquid moving around on the hot surface for little bit like everything is just fine and dandy.
This video was made as a promotional piece for Modernist Cuisine. The Leidenfrost Effect matters for cooking because it allows you to tell when you have successfully heated up a pan. If the temperature of the pan is above the Leidenfrost point, then you can sprinkle it with water and watch the droplets bandy about on the hot metal. In this case, though, they used liquid nitrogen.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.