This is a pretty subtle phenomenon that sometimes goes by the name of the "Leidenfrost effect." Here's how it works:
If the pan is not too hot, the water lands on the surface and absorbs heat. That heat will make it evaporate or, if the surface is hot enough, may even make it boil. Either way, the important thing is that the water makes direct contact with the pan and stays in contact.
If the pan is very hot, then as the drop of water hits the surface some of it can boil off rapidly, forming a sort of cushion of steam that insulates the droplet from the hot surface. The water droplet is now in contact with hot steam, which is much less able to heat the water than the hot surface of the pan. As the steam escapes from beneath the droplet, the droplet gets closer to the pan and more of it vaporizes, restoring the cushion. This means that as long as the droplet isn't too small (i.e. so it can afford to lose some water to maintain the steam cushion), it will be able to hold itself aloft for quite a while - tens of seconds.
This production of steam is also why one wants the pan so hot for pancakes. The steam produced when the batter hits helps prevent it from sticking and burning; almost paradoxically, a cooler pan has a better chance of burning the batter than a really hot one!
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.
