Certain stones, or toys, can change the direction of their spin without being touched. They spin one way, slow down, stop, and then spin the other way. How is that physically possible? Find out.

The toys have a lot of different names. Rebellious celts, celts, rattlebacks, wobblestone, spin bar, space pet, bizzaro swirls; the list goes on. One thing they all have in common is the fact that, when spun on a table, they start rocking. Eventually, they stop spinning entirely, only rocking back and forth, and then they spin the other direction. Some will spin one way, but not another. Others will just switch off. How on earth do they do that?

It turns out, that's really hard to describe. Really. However, everyone makes clear is that the rattleback has to have two curves at the bottom, a long smooth lengthwise one, and a sharper one across the width. Mass has to unevenly distributed along these two curves. The interplay of these curves and the mass results in an uneven application of frictional force. One direction has to fight against more friction than the other. Probably the clearest way to see a rebellious celt at work is this video below:

You'll notice the video makes a point of showing that the handle of the spoon is bent so that the spoon is unevenly balanced when it is spun. Most people have experience with this. Anyone who has spun around carrying a bag or a heavy object in one hand (or on thier back), knows they feel a little extra kick of force in one direction. Since that kick is diagonal to the axis of spin, it makes the spoon move a little too fast that way, rocking up, until gravity causes it to rock back. This motion slows the spoon down.