A rattleback is an object that rotates in only one direction.
If you try to rotate it in the "wrong" direction, it stops and begins
rotating in the other direction.
There are no internal parts .. no mechanism .. to produce the effect.
Rattlebacks can be made from wood, stone, bone, spoons, chewing gum, or
any ridgid object.
The toy version is made from clear plastic.
These objects have been found in Celtic archelogical sites.
The "trick" is that
The length is significantly longer than the width
The center of dynamic (rotational) balance is different than the center of static balance
The bottom surface is not symetrical
Another type of rattleback works by having off-axis weights. In this case
The length is significantly longer than the width
The bottom surface is symetrical
The bottom symetry is not alligned with the weight distribution
When the object rocks, the side with the weight falls slightly farther than the
low mass side, and this produces a twisting motion.
For a rattleback to work, the point that touches the surface
when it is not spinning is different from the point
touching the surface when it is spinning.
There are 2 basic ways to make rattlebacks
The part that touches the surface has an asymetry in it so that rocking motion
causes rotational force
Weights are used to separate the static and rotaional balance points.
If you tap the end of a rattleback, it will start rotation in one direction.
If you tap it in the middle, it rotates in the other direction.
By the way, some rattlebacks will change directions several times.