Ferrofluid is a colloidal suspension of ~10 nanometer magnetite nanoparticles in oil, developed by NASA researcher Ronald Rosensweig in the 1960s for pumping fuel in zero gravity. With no field applied it stays a smooth, ordinary liquid. Once an applied magnetic field exceeds a critical strength, magnetic energy overcomes surface tension and gravity, and the surface spontaneously buckles into a regular hexagonal lattice of spikes — the Rosensweig instability. The spacing between spikes follows the Rosensweig dispersion relation, set by fluid density, surface tension, and field strength. The transformation is fully reversible: reduce the field and the spikes relax back into a flat pool.