The Leidenfrost effect is a phenomenon of physics in which a liquid, close to a surface that is significantly hotter than the liquid’s boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly.
This is most commonly seen when cooking, when a few drops of water are sprinkled onto a hot pan. If the pan’s temperature is at or above the Leidenfrost point (approx. 193 °C or 379 °F), the water will skitter across the pan and take longer to evaporate than in a cooler pan.
On commenter on Reddit noted how the phenomenon used to freak him out as a child. His parents owned a diner and his mom cooked. “She would grab a handful of cut french fries from a bucket full of potatoes and water, shake them once, and then stick her hand and the french fries in the fryer – never getting burnt.”
The effect has also been used in some potentially even more dangerous demonstrations, such as the one below. It is responsible for the ability of a person to quickly dip a wet hand or finger in molten lead or blow out a mouthful of liquid nitrogen without injury. The latter is potentially lethal though, particularly should one accidentally swallow the liquid nitrogen. So don’t try this at home!