I made this dual sphere puzzle as my fourth entry to the Brain Teaser Puzzles contest.

To “solve” it the small sphere should be inside the bigger one. Easy to understand a little bit less to build.

You can play with the dimensions (parametric table).

The actual dimensions are maybe a little bit too tight, so a little bit of force could be used in rotation (yes that's a hint).

To make a full puzzle you need:

• 2 x half moon.
• 1 x of everything else.