This photo was sent to me by Christian Blohmann from the Max Planck Insitute for Mathematics in Bonn. It was taken in the hallway outside his office, which is often used by passers-by on their way to and from seminars, afternoon tea, etc. This particular board had been used as a backdrop for an interview of Professor Don Zagier, one of the directors of MPI and a very distinguished number theorist. Christian particularly liked the photo as it shows a kind of modern mathematical palimpsest, with many different layers of mathematics on top of one another. Mathematicians would often rather pick up a different coloured pen and start writing straight away than take the time to clean the board!