Why do moving water droplets form on the surface of a very hot pan? https://www.youtube.com/watch?v=jrtyTN4aHmo&feature=youtu.be
The above video is my recording of the phenomenon.
Why does this occur at high temperatures, when at lower temperatures the water vaporises upon contact with the hot surface of the pan? Note that there was no oil or other substance on the surface of the pan.
 A: Easy :  since the pan is very hot, there's part of the water that vaporizes immediately under the droplets, and make a sort of insulator.  The droplets are floating on a layer of vapor, so they don't touch the pan and don't vaporize immediately.  The vapor makes the surface frictionless, so the droplets could slide freely on the pan, like if there was some oil.
Related in some way :  think about this.  To get some slidding skiies on the snow, you add some sticky wax on the bottom part.  Why does it make the skiies slide better on the snow, while wax is actually "sticky" ?  This apparent paradox is almost a Zen koan !
A: This is called the Leidenfrost effect.
This happens when the water touches the hot pan and a layer of water vapor forms between the interface of the liquid and the pan which insulates the water against the heat of the pan.
This doesn't happen at lower temperatures because the rate of vapourisation is not enough to form an insulating layer and therefore the water just plainly vapourises.
The same thing happens with liquid nitrogen when it comes in contact with surfaces at room temperature.
