The horizon is always flat, so the Earth must be flat
Flat-earth claim
The claim
"The horizon is always perfectly flat, no matter where you stand or how high you go. A flat horizon means a flat Earth."
The reality
The horizon looks flat because you are far too close to the surface to see the curve with your eyes - but the geometry is real and measurable. The curve is not missing; it is just gentle at human scale. From an airplane the horizon is hundreds of kilometres away, and from the ISS the Earth's limb is unmistakably curved.
The evidence
Earth's equatorial diameter is 12,756 km (NASA). Horizon distance follows d = √(2·R·h): at 10 km (cruising altitude) the horizon is ~357 km away, and the surface there sits only ~8 m below your horizontal - too subtle to see, exactly as observed. At 400 km (ISS altitude) the horizon is ~2,258 km away and Earth spans ~140° of your field of view - clearly curved, as in the photo.