I’ll have to make a blog with all the reasons I can think of sometime. For now I’ll give you the first that comes to mind that anyone can try.
Mountains (Or just the horizon). For two reasons:
- [Smart-ass logic:] A flat earth lies in one plane with two dimensions so it can’t have bumps. Mountains are bumpy.
- [Empirical logic:] Look at a mountain afar and you see mostly the top but as you get closer you see more and more. That suggests a curving of the land — a convex curvature. Same logic applies for the horizon and how clouds far away are apparently touching the ground.
Just look near and far and you’ll fail to see flatness.