Ferrofluid, as described here in greater detail, is a suspension of iron oxide nanoparticles that are so small, that they align themselves immediately with an applied magnetic field (superparamagnetism). This results in a spiky pattern that gives an indication of the position of magnetic field lines, which I like to refer to as the ‘hedgehog’.

If you inject a water based paint into the ferrofluid, when a magnet is held on the other side, you will be able to form domains of colour suspended in the fluid – as seen above.