Summing to zero (2016)
Concrete cast meissner tetrahedra
These forms don't look like they should roll like a ball, but they do! the distance between the surface they're rolling on and their top is always constant – so they're called a shop of constant width. That means they can work (unintuitively) as ball bearings. The middle picture is me rolling a board evenly across them– the board stays totally flat as they roll!