Some materials have a characteristic known as ferromagnetism. The prefix "iron" refers to iron, which is one of these materials. Ferromagnetic materials have the ability to "remember" the magnetic fields to which they have been subjected. An atom is made up of a number of negatively charged electrons, orbiting a positively charged nucleus. These electrons also possess a quantity known as spin, which is roughly analogous to a spinning top. The combination of orbital and spin motions is called the angular momentum of the electron. Angular momentum is perhaps most easily understood in the case of the Earth: the Earth spins around a central axis, which means it has angular momentum around that axis. Planets also have angular momentum as they rotate around the sun. Now, the angular momentum of an electron is a vector quantity, meaning it has a direction. The movement of the electron produces a current, which in turn generates a tiny magnetic field in the direction given by the angular momentum. So an atom can behave like a dipole, i.e. "two poles". The direction of the electron's orbital and spin angular momentum determines the direction of the magnetic field of the electron and the entire atom, thus giving it the "north" and "south" poles. Different atoms have different arrangements of electrons in their orbits and therefore have different angular momenta and dipole properties. A ferromagnetic material is composed of many microscopic magnets known as domains. Each domain is a region of the magnet, made up of numerous atomic dipoles, all pointing in the same direction. A strong magnetic field will align the domains of a ferromagnet, or in other words, magnetize it. Once the magnetic field is removed, the domains will remain aligned, resulting in a permanent magnet. This effect is known as hysteresis. Few materials are actually ferromagnetic; however, all substances have a diamagnetic nature. Diamagnetism means that molecules within a substance will align themselves with an external magnetic field. The external magnetic field induces currents within the material, which in turn give rise to an internal magnetic field in the opposite direction. This effect is usually quite small and disappears when the external magnetic field is removed. Some materials are paramagnetic. This is the case where the orbital and spin movements of the electrons in a material do not completely cancel each other out, so that the individual atoms behave like magnetic dipoles.