A fridge magnet sticks strongly to steel but not to an aluminium drinks can. Being a metal is not enough to make a material strongly magnetic. Iron, nickel, cobalt and many of their alloys contain magnetic regions called domains. In an unmagnetised piece these domains point in many directions, but an external magnetic field can make more of them line up. That produces a strong attraction. Materials such as aluminium respond much more weakly under ordinary conditions, so a fridge magnet will not cling to them. The behaviour depends on the material's microscopic structure, not simply on whether it conducts electricity.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Magnetism comes from magnetic fields, which exert forces without objects needing to touch. In magnetic materials, many tiny magnetic regions can become aligned, producing a larger overall field. Electric current also produces a magnetic field, which is why coils of wire can act as electromagnets and why magnetic effects can be controlled in motors, speakers, relays and sensors.