I recently went to a county fair. From the ferris wheel, I could plainly see the curvature of the earth at the top. This is true for tall buildings as well. How can this be possible if the Earth is flat?
That's an optical illusion. You have to get above something like 80,000 ft. to start perceiving the earth as a sphere. (Being a subtle human perceptual thing, there's no one magic number.) But even then, you are only seeing a tiny, tiny fraction of the globe.
(Photos from planes and amateur rockets only confuse the issue - for both sides of the "debate" - with wide-angle lenses that exhibit barrel distortion. [When the horizon is above the center of the image, it looks more round. When below, it looks less round or concave.] Anytime someone from either side of the debate submits such a photo as proof, consider the possibility that they don't have a good grasp on optics, and/or are talking out of their large intestine.)
This inaccurate misconception is the flipside of the inaccurate misconcption that leads some people into accepting one or more of the FE hypotheses.
Here is a good explanation of how much of the globe you can see - and addresses one of the most common and idiotic FE "debunkings" of the globe model (e.g. "round-earthers are so incompetent that they can't even make the relative sizes of continents consistent").: