FERROFLUIDS are a suspension of magnetite particles in a medium in such a way that each magnetite molecule does not touch another.

it has many uses in sound tech,space tech etc. since application of a magnetic field can change it's apparent density.

here's a cool video showng magnetic fields acting on ferrofluid
http://video.google.com/videoplay?docid=7932498063864415301






BTW i made some ferrofluid myself....