You might have learned how to tell when your pan is hot enough in home-ec class: just observe a water droplet or two on the pan's surface and wait for it to bead up and roll across the hot pan. But did you know this trick has a name?
It's called the Leidenfrost Effect. Ideally, you want a mercury-like ball of water to hover over the pan, which happens at 320°F or the Leidenfrost point. The water should evaporate more slowly than it would at lower temperatures but if many tiny bubbles form, that means the pan is too hot.
This neato two-minute video from Rouxbe, the online cooking school, explains the very good life knowledge. Watch it, after the jump.
How to Preheat Your Pan
[via Houseboat Eats]