Here we are making a 3D-printable object made with 2 pieces that fit together.

I decided to make a hollow wheel with a sphere inside that can roll around inside of it without falling out.

Here’s my (hacky) sketches:

Decided to make it out of a bunch of interlocking toruses so that the sphere would be more visible as the wheel rolls.  Here’s the model:

Hopefully the effect of the ball flowing through the wheel looks cool and relatively smooth.