London 2012 Olympic Football 50p (2011)

The rarest of the 29 Olympic and Paralympic sport designs, with 1,125,500 struck for circulation, and the only one that teaches you something. Its designer, sports journalist Neil Wolfson, explained the offside rule in simple shapes: a circle for the ball, triangles for the attacking team and squares for the defenders, under the words Offside Explained. Every coin in the series came out of a Royal Mint design competition open to the public, and this is the one that usually stands between a collector and the full set.