Here's why:

* Earth's Magnetic Field: Earth has a magnetic field that acts like a giant bar magnet, with a north and south pole. This field is constantly shifting and can even flip completely, reversing the magnetic poles.

* Magnetized Rocks: Some rocks contain magnetic minerals like magnetite. When these rocks form, they align themselves with Earth's magnetic field at that time. This process is called paleomagnetism.

* Reversals and Records: When Earth's magnetic field reverses, the newly formed rocks will have their magnetic minerals aligned with the reversed field. This leaves a record of the past magnetic field.

Therefore, if a rock has a magnetic field pointing south, it means it formed during a time when Earth's magnetic field was reversed. Scientists can study these rocks to understand the history of Earth's magnetic field and its flips.