Also known as a celt (pronounced “kelt”). A colorful piece of plastic that is 3” long and, at first blush, resembles a hot dog bun. When you place the celt on its rounded side and spin it counter-clockwise, it just spins and spins until it stops. If you choose to spin it clockwise, it will spin for a couple of seconds, stop, rattle back and forth a couple of times, and then begin to spin counter-clockwise. These occur in nature as smooth rocks found in fast moving streams. Some folks try to explain it using chaos theory; I prefer to simply win cheeseburgers from unsuspecting passers-by.
Demos and Labs: Rattleback, Carry Me Back