Wikipedia defines the Leidenfrost effect as “a phenomenon in which a liquid, in near contact with a mass significantly hotter than the liquid’s boiling point, produces an insulating vapor layer which keeps that liquid from boiling rapidly.”
Reading the concept might be boring, but seeing it in action is somewhat cooler. In this little clip, a glowing ball of red hot nickel is dropped into a container of water. Thanks to Leidenfrost effect, the surface of the ball becomes insulated from the water by a blanket of steam. But the effect is temporary, watch what happens when the ball cools.
The Leidenfrost effect has been demonstrated in a few other ways, most notably when the mustachioed Mythbuster Jamie Hyneman dared to dip his little piggy into a pot of molten lead. Have a look at that reaction after the jump.