Summary Inspired by the Disney Research paper on balancing toys with movable masses I created my own L-pipe toy to experiment with the technique. The STL has a pocket for a 16mm diameter marble The print must be paused between 27mm and 30mm for insertion of the marble. Check out the paper here: https://www.disneyresearch.com/publication/balancing-3d-models-with-movable-masses/ Update: I uploaded the wrong STL version initially. That has been corrected. as of Oct. 10 2016. Print Settings Supports: Yes Notes: Pause between 27mm and 30mm print height to insert the marble.