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Soldiers may no longer play football in no-man’s-land, but that hasn’t
stopped Britain’s Defence Evaluation and Research Agency developing a smart
football. The ball tells you when it’s inflated to the correct pressure (GB
2353864). It has a patch in its surface in which two films are separated by an
opaque gel. The top film is transparent, while the lower one is embossed with a
diamond and a cross, with the cross standing more proud than the diamond. When
the ball is under-inflated, the gel spreads thickly and neither cross nor
diamond is visible. At the correct pressure, the cross alone touches the top
film, but over-inflation squeezes the films so tightly that both shapes are
visible.