Try using only one or two magnets. As the magnetic field strength gets weaker, it gets harder to keep the paper clips from falling. How could you modify the sandwich to make a weaker magnet work better?
Insert an open pair of steel scissors between the pieces of cardboard, then close the scissors as if cutting the field lines. The paper clips will drop off when the field lines interact with the closed scissors.
Scientists model magnetic forces using field lines. A compass lines up along a field line, so you can trace field lines with a compass.
Iron filings also line up along field lines. By sprinkling iron filings on a piece of paper placed on top of a magnet (or a couple of magnets), you can see the field lines. Our Snack, Magnetic Lines of Force, demonstrates this phenomenon.
Field lines come from the north pole of a magnet and return to the south pole. Magnets have field lines that exert a force on magnets or iron. If there are no field lines, there is no force, and the result is magnetic shielding.