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Alexander Graham Bell (1847-1922), demonstrating the uses of tetrahedral cells at an exhibition. Bell is most famous for inventing the telephone, but in later life became increasingly interested in engineering structures. He championed the cause of tetrahedral cells, a series of interlocking frameworks of equilateral triangles based on the regular tetrahedron. These had a very high strength to weight ratio, but never really caught on in the late 19th century. It was not until the mid-20th century that tetrahedral cells became popular as a lightweight structural component. Bell's partner in this research was Frederick 'Casey' Baldwin. (KEYSTONE/SCIENCE PHOTO LIBRARY/LIBRARY OF CONGRESS/SCIENCE PHOTO LIBRARY)