Store-bought compasses have a lightweight and magnetized pointer fixed on a low-friction pivot; a small plastic cylinder filled with liquid seals the pointer. Settle the needle and rotate the compass card (the layer of plastic encasing the cylinder) until the needle lines up with the north/south axis label.
The compass card comes labeled with the cardinal points (north, south, east, and west), which clearly provide the navigational markings. The magnetized pointer finds these points in its pull toward the Earth's magnetic field. In the Northern Hemisphere, a compass needle attracts the magnetic south pole, and vice-versa, because unlike poles attract.
However, Earth's magnetic field is relatively weak compared to gravity and friction. For the needle to respond to the earth's magnetism, these more substantial forces must have a lower threshold: the needle must be lightweight and mounted with minimum frictional resistance. To establish an intended direction or the cardinal points, the magnetic force should have less gravity and friction to overcome.
Common problems for compasses include a crazed or cracked dome, where this threshold suffers damage from increased pressure. The damage can result from a fall or excessive sun exposure, which usually leads to a bubble and indicates more gravitational and frictional influence; thus, needle's sensitivity to the magnetic field lessens and the compass becomes inaccurate.