I noticed it at my mother’s house over halloween, and am just now getting around to asking about this.
It’s a plastic witch with a cauldron. You fill the cauldron with regular tap water and plug it in. You can hear a little motor which circulates the water.
In minutes, dense heavy fog covers the water in the cauldron and spills over the sides. The fog is identical in appearence to fog produced by dry ice.
There is no place to add anything else besides water, unlike a stage smoke machine.
I remember an ultrasonic humidifier that we used to have when I was a kid, that produced similar fog when opened up. It had a stream of water rising up out of the middle of the reservoir of water … and when you put your finger in that stream of water, it stung like crazy.
Also … heh heh … this was really stupid … when I put my glasses in the stream so that the stream of water hit the front of the lens, after a few seconds the stream of water would rise straight through the lens and continue to its ordinary height. I called my brother and sisters and parents in to see how cool that was, and they speculated that the stream was somehow continuing around the side of the glasses and pooling on the top of the lens, and that the ultrasonic beam was then pushing the pool of water up somehow.
As it turned out, of course, the water beam was melting right through the lenses – which we had forgotten were plastic – and my glasses were full of warped circular blobs. Not only did I have to get new lenses, I had to endure the mirthful laughter of the optometrist and his entire staff as my mom explained what had happened. Grrr…
As others have said, it’s ultrasonic: an underwater ceramic transducer vibrates the water into small droplets. This may be the witch centerpiece you mean; and here, more about the technology of ultrasonic misters.