Drip water on a hot pan, and the droplets will skitter around the pan, speeding like tiny mad hovercraft on cushions of steam.

This is the Leidenfrost effect, which you’ve probably experienced while cooking. Johann Gottlob Leidenfrost, a German doctor and theologian, described the phenomenon in 1756 in a book about the properties of water.

But French scientists have now figured out something new about those skittering drops. When they are small enough — about a millimeter in diameter — the roiling of heat in the liquid will cause the droplet to tilt and rotate. That, in turn, propels the droplet to roll.