Cork Plug

A 3-D Solid which can stopper a Square, Triangular, or Circular Hole. There is an infinite family of such shapes. The one with smallest Volume has Triangular Cross-Sections and $V=\pi r^3$; that with the largest Volume is made using two cuts from the top diameter to the Edge and has Volume $V=4\pi r^3/3$.

See also Stereology, Trip-Let