Day 4
A book with a worldbuilding detail that has stuck with you.
Despite being an atheist, I really like the Narnia books (except the Last Battle) and consider them just as much fantasy classics as the Lord of the Rings. One fascinating bit that shows up in Voyage of the Dawn Treader (which I always thought had one of the more interesting plots of the books) is that not only is Narnia geocentric rather than heliocentric, it's a flat world!
