Here is something that everyone has seen many times. A round-earth believer, or shape denier as I will call them from here on out, posts a pic from high altitude or makes the statement that in a plane you can see the curvature of the earth. And indeed, the pictures of the shape deniers do look compelling. However, it's all an optical illusion.
At ground level, if you look out the window of a plane every flat-earth believer and every shape denier can agree that the earth looks flat. However, at extreme altitudes, it appears that there is a curvature. This is well documented and understood. Now, a shape denier will point to this and ignorantly proclaim, "Ah hah! We've got you! Another win for RET!!!" Seems like a solid claim on the surface, lets look deeper.
The traditional response to this is that they are seeing the outline of the sun's spotlight, and at very very high altitudes or close to dawn/dusk this is true. However, some pictures are taken at noon, or at the center of the spotlight at altitudes that shouldn't show curvature. So what's going on?
The real reason that the earth looks curved is due to air pressure. In order to keep passengers and personnel comfortable, and alive, a plain must maintain a certain atmospheric pressure. Once a plain gets so high, the pressure outside the fuselage is lower than it is on the inside. This causes the windows to bow outwards and give objects in the distance a curved look. As the horizon is the only thing viewable that spans the length of a window, and at a sufficiently far distance, it appears to be curved.
This optical illusion has fooled many people the world over, but it will no longer deceive you! The next time you are on an airliner and a kid points out the window claiming to see the curvature of the earth, you can educate him or her on what is really going on and hopefully open their eyes to truth of the shape of this flat earth.