I love flat-earthers because I feel like their bullstuff works well as an exercise in using rational thought to debunk nonsense.
1. Horizon always looks flat
https://teachingcalculus.com/2012/08/29/local-linearity-i/Plus, there is high-altitude footage of the Earth's curvature, but they obviously deny its legitimacy since it undermines their beliefs. Not just from NASA either.
https://phys.org/news/2017-01-students-breathtaking-curvature-earth-high-altitude.html2. Don't have to tilt your head down to see the horizon
This is basically just linear perspective. The distance your head has to point down is a triangle subtended over a ridiculously long distance. You do have to tilt your head down, but only by a marginally small degree.
3. Natural physics of water says water should just be flat, not curved
Only in a system where gravity points down at every point. If you take a magnet and stick ferrofluid on it in zero-G, it forms what's more-or-less a sphere.
Etc etc and-so-on x197 times.