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Why does a drop of water dance on a hot pan?

Q: When my mom makes pancakes she tests the pan to see if it's hot enough by throwing drops of water on it. If the iron is hot enough, instead of boiling away, they skid around for a long time. But that doesn't make sense -- shouldn't they boil away faster if the plate is hotter?
IS, Boston

A: The surprising phenomenon you describe is called the Leidenfrost effect, after the German physicist who described it.

The idea is pretty simple. If the pan is really, really hot, then when a drop hits the bottom, it can vaporize so fast and hard that it pushes the droplet back up and off the pan. The drop is now insulated from the hot bottom by a layer of steam, and can skate around on that layer for a long time, because steam does not conduct heat very well.

Even more dramatic effects have been demonstrated by physicists like Jearl Walker, including putting one's hand quickly and briefly into a pot of molten lead. Water on the skin from perspiration (and I'm quite sure anyone who would try such a dangerous stunt would be nervous enough to be sweating!) vaporizes quickly and produces a protective jacket of steam that prevents the molten lead from touching the flesh. Of course if you leave your hand in too long, don't have enough water on it, or didn't heat the lead enough, you lose your hand -- so don't try this at home.

The effect may also play a role in the famous ``firewalking" events where people can walk quickly across hot coals without being burned. At least part of the effect seems to be the formation of protective cushions of steam as one runs across the coals -- it's not much, but perhaps just enough to help prevent hard and prolonged contact with the coals.

Dr. Knowledge answers your questions about science each week. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. Include your initials and hometown.

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