Why do water drops skitter on a hot pan?

The moment a water drop touches very hot metal, only its underside turns to steam. That steam forms a thin layer between the drop and the pan and lifts the drop slightly. Steam carries heat far worse than metal, so heat enters the water more slowly than if it touched the pan directly.

With almost no friction underneath, the drop beads into a small ball and glides around. It is a bit like a puck sliding on the cushion of air on an air hockey table.

Below 100°C, water spreads out and dries slowly. Above 100°C, it hisses and vanishes quickly. Then, at roughly 190°C or so for water, a stable steam layer forms and the drop rolls around like a ball.

This threshold is not one fixed number. It shifts with the condition of the pan's surface and what is mixed into the water; on copper, values above 250°C have been reported.

It is a common misconception that water always evaporates faster on a hotter pan. In an experiment cited by the 19th-century British boiler designer William Fairbairn in his lectures, a drop that vanished almost at once on iron at 168°C lasted 152 seconds at 202°C.

That made the effect a headache for boilers: when the iron got too hot, less heat actually reached the water. Once the pan cools and the steam layer breaks, the drop boils hard again and disappears fast.

Cooks who use stainless steel pans treat this as a preheating signal. They heat the pan slowly over medium heat and add a little water now and then; when the drop pulls together into one bead that rolls like a ball of mercury, the pan is ready. Wipe away the water, then add oil.

Cast iron has a rougher surface, so the bead may not form as cleanly. In that case, judge by the heat rising from the pan and the sizzle when food goes in.