Recall the "napkin ring problem": Take a sphere of radius r and drill a hole along a diameter so the resulting shape has height h. Then the volume, V = πh³/6, does not depend on r!

These files consist of drilled spheres of the type described in the problem. Each has a height of 4cm, but the radii are 4.15 cm, 5 cm, 7 cm, and 10 cm. It is essential that you print them with 100% fill. If you do, you will find that they all weigh the same amount and hence have the same volume.

Thank you to Colm Mulcahy for suggesting this project.