Petri dish containing ferrofluid on a powerful permanent magnet. A breeze is used to disturb the spiky shape the fluid takes in the field, but although individual spikes are disturbed, the overall pattern remains. Ferrofluid is a suspension of iron_rich nanoparticles in a carrier oil. In the presence of a magnetic field the iron particles become magnetised, attracting each other and following the shape of the magnetic field. This leads to the formation of spikes of liquid showing the magnetic field lines.