I was reading an article about anomalous time zones [1] (link via Hacker News [2]) and when it mentioned an area around the International Date Line [3], I couldn't visualize how the International Date Line actually works.
In fact, I never really understood how it worked.
So I'm sitting there, reading about it and it doesn't make sense to me. Even when I picked up a globe and shone a flashlight on it (to simulate the sun) and spinning the globe, I still couldn't figure it out. I eventually found an animated visualization [4] that worked.
Finally!
[1] http://www.hopesandfears.com/hopes/city/city/215293-the-time-zone-rebels-of-the-world
[2] https://news.ycombinator.com/item?id=9957979
[3] https://en.wikipedia.org/wiki/International_Date_Line