Flick a little water onto a warm frying pan and it sits there, steaming gently. Turn the heat up and the next drop hisses, spits and is gone in a moment. So far, so sensible: more heat, faster boiling.
Now keep heating. At some point the rule breaks. A drop lands, pulls itself into a shiny bead and skates around the pan like a ball bearing, silent and unhurried. It can survive for a minute or more on metal that would have destroyed it in a blink when the pan was cooler.
Try it here first. Set the pan temperature, drop some water, and watch how long each drop lives.
If you filled in the chart you will have found a cliff and then a wall. Lifetime falls as the pan warms, crashes to almost nothing, then leaps up by a factor of hundreds.
The drop spreads into a flat puddle and evaporates from its surface, like a spill drying on a warm day.
Water touches metal directly. Bubbles burst out from underneath, the drop hisses and it is gone in a second or less.
The underside flashes to steam so fast that the drop never lands. It rides on a cushion of its own vapour.
The third regime is the Leidenfrost effect, named after Johann Gottlob Leidenfrost, a German doctor who described it in 1756 in A Tract About Some Qualities of Common Water.
The trick is that the drop builds its own insulation. As it approaches very hot metal, the bottom layer of water vaporises before the rest can touch down. That vapour has to escape sideways, and while it is squeezing out it holds the drop up, the way air holds up a hovercraft or a puck on an air-hockey table.
The gap is tiny, on the order of a tenth of a millimetre, but it changes everything. Steam is a far worse conductor of heat than metal, or even than liquid water. Once the drop is no longer touching the pan, heat has to creep across that film of gas, and the drop evaporates slowly from below instead of exploding into bubbles.
The hotter pan loses, because it never gets to touch the water.
The same film removes almost all friction. Nothing is in contact with anything, so the bead glides at the slightest nudge and rebounds off the rim. Surface tension, with no pan surface pulling the water flat, rolls it up into a ball.
For water on a pan, the switch happens at roughly 193 °C. That figure is an approximation: the exact point shifts with how rough or clean the surface is and what is dissolved in the water, and physicists still cannot predict it from first principles.
The jump across it is startling. In one often-quoted measurement, a drop that vaporised almost immediately at 168 °C lasted 152 seconds at 202 °C. Thirty-four degrees hotter, and it lived for two and a half minutes.
Push the temperature higher still and the lifetime slowly shortens again, as you can see on the right side of the chart. More heat does cross the gap. It just never gets back to the frantic pace of direct contact.
In good cooking. Cooks use the dancing bead as a thermometer. When a splash of water balls up and rolls around a stainless steel pan, the pan is hot enough to sear.
In liquid nitrogen. Nitrogen boils at −196 °C, so an ordinary floor is, by its standards, a ferociously hot pan. Spilled drops skitter across the room on their own vapour for exactly the same reason.
In factories. Here the effect is a nuisance. Plunge red-hot steel into water and a film of steam wraps the metal and insulates it, so at first it cools much more slowly than you would expect. Engineers who quench metal or cool hot machinery have to break that film to get the heat out.
In drops that climb hills. In 2006 a team led by Heiner Linke at the University of Oregon put Leidenfrost drops on a hot surface cut with tiny sawtooth ridges. The escaping vapour was steered one way by the teeth, and the drops drove themselves along at about 5 cm per second, even up a slope.
Heat a pan past about 193 °C and water stops boiling on it. The bottom of each drop turns to steam before it can land, and that thin film of vapour both lifts the drop and insulates it. Hotter pan, longer-lived drop: one of the few places in a kitchen where turning up the heat slows things down.
If you try this at home, use a dry, empty pan and a few drops only. Never add water to hot oil.