The Leidenfrost effect is 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. This works because, at temperatures above the Leidenfrost point, when water touches the hot plate, the bottom part of the water vaporizes immediately on contact. The resulting gas actually suspends the rest of the water droplet just above it, preventing any further direct contact between the liquid water and the hot plate and dramatically slowing down further heat transfer between them. This also results in the drop being able to skid around the pan on the layer of gas just under it.*

*Wikipedia entry on "Leidenfrost effect" (less info)