Lord Kelvin posed the question of how space could be divided into cells of equal volume and minimal surface area. This is equivalent to asking what shape equal-sized soap bubbles would take in foam since bubbles naturally assume shapes with minimal surface area. He conjectured that the solution was a lattice of truncated octahedra with slightly curved hexagonal faces. The polyhedral version of this structure is called the Kelvin structure. The physicists Denis Weaire and Robert Phelan applied a structure known from crystallography to the problem and found that the so-called Weaire-Phelan structure improves on the Kelvin structure.