HOW COME?: Dancing water drops

What causes water to sometimes bounce across a hot pan, rather than sizzle away? -- A reader

When a drop of water dances across a frying pan, it's really flying -- the drop is actually suspended above the surface.

And thanks to a 16th-century German medical doctor, the phenomenon has a name: The Leidenfrost Effect. Johann Gottlob Leidenfrost studied the dancing-drop behavior, dripping water onto a red-hot iron spoon, then measuring how long each drop lasted. He reported on the results in 1756, in a work called "A Tract about Some Qualities of Common Water."

Of course, the Leidenfrost Effect is the opposite of frosty. In fact, it's a good way to check whether a frying pan or griddle is hot enough for a pour of pancake batter. Drops of water sizzle and disappear? The griddle's not ready. Drops bounce smartly across the surface? It's hotcake time.

What's puzzling about the effect is why it should occur at all. If a pan is hot enough to make water evaporate into thin air, then why does making it even hotter help water last longer? And why do the drops skitter across the surface?

When a pan's temperature is below water's boiling point -- 212 degrees Fahrenheit -- a tossed-in drop simply spreads out, gradually evaporating. As the surface temperature rises just above 212 F, a dripped drop quickly sizzles away. But keep the pan heating on the stove, and when its temperature reaches about 380 F, you'll be able to see them: Leidenfrost's dancing drops.

How does it work? When water is sprinkled into a hot-enough pan, the bottom of each drop will immediately evaporate -- forming a cushioning layer of steam. Then, like a passenger on a magic carpet, each drop will levitate on its own vapor, flying across the surface of the pan for many seconds.

How do the drops survive? When a drop rides on its own steam carpet, it is held about .1 millimeters to .2 millimeters above the hot metal. This keeps the drop cooler, so it can dart across the pan instead of simply disappearing.

Each tiny hovercraft persists as long as water continues to vaporize from the drop's underside. Eventually, the drop vanishes. Heat the pan too long, and the Leidenfrost Effect will vanish, too. At temperatures hotter than about 428 F, sprinkled-in water evaporates immediately.

But the Leidenfrost Effect doesn't just apply to water and hot metal. Other liquids also can dance. Take nitrogen, the gas that makes up about 78 percent of the air we breathe. Spill some liquid nitrogen in a 70-degree room, and the floor will stand in for a hot frying pan. Nitrogen drops will skitter across the room, riding their own vaporous cushion until they disappear into the air.