crickety - One of the quickest ways to understand special relativity is to probably read a quick overview on time dilation.
For many people, it's easier to understand examples than to understand theoretical concepts or formulas. Therefore, if you read about a simple example that covers the most important fundamentals, it makes it much easier to understand for people who do not work/study in physics.
There are also several good videos on Youtube that explain Time Dilation, for example:
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Let me know if that helps.
Regarding the Earth's core (important: it's the inner-core itself being discussed above, not the outer-core; the outer core is many magnitudes more responsible for geomagnetism) - for that topic you can read about "Earth Dynamo effect" or "Geodynamo". Here's another quick video that will help you understand that:
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I hope these help; if you're interested in the topic, I suspect you can find a good DK book on the topic that will take you from zero to understanding important fundamentals very easily (as with most DK books which are excellent / easy to read / hundreds of visual metaphors & photos, etc).
Good luck to you!