I was reading this paragraph and it got me thinking:
The closed ends of the honeycomb cells are also an example of geometric efficiency, albeit three-dimensional and little-noticed. The ends are trihedral (i.e., composed of three planes) sections of rhombic dodecahedra, with the dihedral angles of all adjacent surfaces measuring $120^o$, the angle that minimizes surface area for a given volume. (The angle formed by the edges at the pyramidal apex is approximately $109^\circ 28^\prime 16^{\prime\prime}$ $\left(= 180^\circ - \cos^{-1}\left(\frac13\right)\right)$
This is hardly intuitive; is there a proof of this somewhere?