system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The Earth's magnetic field periodically reverses polarity. How does this happen and what are the effects felt on Earth?
gpt
The effects are relatively unknown. We know a good number of species rely on magnetism to navigate, so there might be a critical moment that could lead to a kill off event where magnetically dependent species just get lost and die. Or they will just notice everything's been flipped. It's hard to test, I've seen experiments involving lobsters, it seemed to show that lobsters where totally loyal to direction of the magnetic North to find better grounds. Though unfortunately it had a serious observer effect that made it a bit unusable. There is no good way to say for sure. 

It happens because our magnetic field is generated by the spinning ball of metal in the earth's core that is totally surrounded by liquid metal. There isn't really anything there keeping it facing one direction or the other. It will almost always be facing geographic North or south due to its spin.