This sphere has a peculiar property: if you toss it in the air it is nearly impossible to spin it around the axis connecting both sides.

The physical phenomenon is called the "tennis racket instability" and the mass distribution inside the wigglyball is chosen to maximize its effect.

I recommend printing the two halves of each wigglyball in different colors, though if you are using a multicolor printer this makes a nice "waste object": the filament that must be purged after each color change is used to construct this object and this produces stripes that illustrate the tumbling nicely.