A ferrofluid is a liquid which becomes highly magnetized in the presence of a magnetic field. The distinctive ‘spikey’ shape of a magnetized ferrofluid is caused by the need to find the most stable shape in order to minimize the total energy of the system, an effect known as the normal-field instability. The fluid is more easily magnetized than the surrounding air, so is drawn out along the magnetic field lines, resulting in the formation of peaks and troughs. However, the extension of the ferrofluid is resisted by gravity and surface tension. The formation of the corrugations lowers the magnetic energy of the system but raises the gravitational energy and surface free energy. When these forces are balanced, the minimum energy configuration is achieved. Because ferrofluids are very easily magnetized (they have an incredibly high magnetic susceptibility), the peaks can be produced using a small bar magnet. Ferrofluids are known as colloidal fluids and are composed of nanoscale ferromagnetic particles suspended in a carrier fluid, usually water or an organic solvent like kerosene, and coated with a surfactant to stop them clumping together in the liquid. A typical composition would be 5% magnetic particles, 10% surfactant and 85% carrier fluid. Ferrofluids are also the focus of current scientific research and have the potential to be used in many medical applications. In magnetic drug targeting for example, where drugs could be enclosed by ferrofluid and, once injected into the specific body area requiring treatment, a magnetic field could be applied to keep the drugs in this target area. The localization would limit exposure to the rest of the body and enable the dosage level to be decreased, reducing the adverse side effects experienced by the patient.