How would bubbles pack together, to give the least possible amount of surface film between them? This question has not been answered yet. In the nineteenth century Lord Kelvin considered the simplest case of identically shaped bubbles. He discovered a packing based on the truncated octahedron but with slightly curved faces and conjectured that this was the most efficient solution. A hundred years went by before Weaire and Phelan found a better one. They asked the slightly less strict question, how would equal sized , but not necessarily identically shaped, bubbles pack? The best answer they came up with was a mixture of 12- and 14-sided bubbles having slightly curved surfaces. We still do not know is this is the optimal solution.