The outer wall is based on the Weaire–Phelan structure, a structure devised from the natural pattern of bubbles in soap lather. In the true Weaire–Phelan structure the edge of each cell is curved in order to maintain 109.5 degree angles at each vertex (satisfying Plateau's rules), but of course as a structural support system each beam was required to be straight so as to better resist axial compression. The complex Weaire–Phelan pattern was developed by slicing through bubbles in soap foam, resulting in more irregular, organic patterns than foam bubble structures proposed earlier by the scientist Kelvin. Using the Weaire–Phelan geometry, the Water Cube's exterior cladding is made of 4,000 ETFE bubbles, some as large as across, with seven different sizes for the roof and 15 for the walls.