Ferrofluid on glass, with a rare-earth magnet underneath. A ferrofluid is a liquid that becomes strongly polarised in the presence of a magnetic field. Ferrofluids are composed of nanoscale ferromagnetic particles suspended in a carrier fluid, usually an organic solvent or water. Ferrofluids do not actually display ferromagnetism, since they do not retain magnetisation in the absence of an externally-applied field.Photo credit: Greg Maxwell/Moondigger