The True Size Of Countries: Mercator projection
I still remember the first time I learned about this. I was watching the West Wing (Season 2 Episode 16[1] which explains the problem pretty quickly) for the first time in the early 2010's and was absolutely blown away that we'd never been taught about this in public school (U.S.A.). Having graduated High School and a couple of years through undergrad, and just then realizing that my entire perception of the relative size, and in some cases position(!), of the continents is basically just wrong. It was actually a generally useful thought exercise though, because it really forced me to confront the fact that I need to consistently reflect on whether my previous assumptions on even basic topics hold true.I'm not sure it would be worthwhile to advocate for changing the default projection at this point due to the logistics, but at least teaching kids about the issue, which can be covered in no more than a few minutes and is obviously not super complex, just so they're aware of it seems totally worth it![1]: https://www.youtube.com/watch?v=vVX-PrBRtTY