When a drop of water hits a hot enough pan, it doesn’t instantly boil away. Instead, the drop’s outer layer vaporises, producing an insulating effect that causes it to skitter across the hot surface. This is known as the Leidenfrost effect and it can be harnessed for some neat tricks, like the Mythbusters being able to stick their hands in molten lead, or this wonderful science experiment from the University of Bath.
When water droplets hit a smooth hot surface they tend to hop about in random directions, but researchers have found that when that hot surface features a saw-tooth-like texture, the droplets are only propelled in one direction. And when contained and heated in an all-metal maze that’s covered with a saw-tooth texture, it appears as if tiny droplets of water are able to navigate its complex design all by themselves.