I think what MC said today  true.I did resaerch and it said:
If iron filings are sprinkled around a magnet, the filings arrange themselves into a pattern of lines. These lines are called line of force, and they show where the magnetic field is located and how it is arranged. There are more lines of force, bunched closely together, at the poles of the magnet, where the magnetic force is strongest.




A magnet can attract magnetic materials (iron, steel, cobalt, nickel) without even touching. This attraction occurs because the force of a magnet can pass through any material that is not magnetic, such as air, water, paper, wood, glass, aluminum, and brass. In other words, magnetic field lines can normally pass through nonmagnetic materials (be it solid, liquid or gas). However, the magnetic field lines does not pass through magnetic materials because these materials hold or keep nearly all the magnetic field lines so that practically none of them can pass through. Magnetic fields behave similarly to electricity in that electric currents travel along conductors due to their low electrical resistance. Hence, you can think of magnetic field lines like magnetic currents traveling along conductors that have low magnetic resistance.