I made a Meissner Tetrahedron using a CAD program for design and then 3D printing.
It's the lumpiest possible shape that still acts as a perfect ball bearing when rolling one flat surface over another. It's a pyramid with a triangular base, rounded faces and half of its 6 edges rounded in a special way.
I made a three different shapes all with the same 'diameter' but the other two are a bit less interesting than the first one as you could cut them in certain directions and get circular cross sections. Now I can balance a large hardcover book on three of these lumpy looking shapes lying on a table then and spin the book around smoothly while it remains at a constant height and perfectly level.
I showed the things to a few friends and they all thought they were cool to begin with but then went on to ask what was the point of making them and did they have any practical use.
