Iron filings being showered over a surface, beneath which is a powerful permanent magnet. The filings are attracted to the magnetic field, and become magentised themselves in it, with each individual filing gaining a north and a south pole. These attract each other, forming chains of filings that reveal the field lines generated by the magnet. The magnetic field extends in three dimensions, as can be seen from the filings standing upright from the surface over each pole of the magnet.