Every so often, however, that regular motion is interrupted. Think of what happens when you stir that bubbling soup. It takes a few minutes for things to settle down and the regular pattern of bubbling to resume. That’s probably what happens inside earth. Something–maybe nothing more than naturally occurring variations in the process– disrupts the otherwise regular flow of metals in our planet. This shuts off the magnetic field for a while. Eventually, things start bubbling away again and the magnetic field returns. When it comes back on, one end will be north and one end will be south. Whether or not they were aligned that way before the disturbance is a matter of chance.