Ferrofluid does not turn magnetic when a magnet approaches. It is already magnetic, and the field simply reveals what the liquid has been carrying all along. The composition is deceptively simple. Particles of magnetite roughly ten nanometres across are suspended in a carrier fluid, usually a light oil or water, and every particle is wrapped in a surfactant such as oleic acid. That coating is the entire engineering problem solved in a single layer. Bare magnetite grains would clump within seconds under their own mutual attraction and settle out as sludge. The surfactant creates a steric barrier that holds neighbouring particles apart, while thermal agitation keeps them permanently stirred. The result is a colloid stable enough to survive years in a sealed bottle. Particle size is not arbitrary. Below roughly fifteen nanometres, magnetite exists as a single magnetic domain and behaves superparamagnetically: a strong response to an applied field, but no residual magnetisation once that...