According to the flat earth wiki, the phenomenon known as sun sets, is produced by the sun, not going underneath the earth, like many/most ancient flat earthers believed, nor the earths rotation, like round earthers believe, but by the sun traveling far away from us, the way clouds travel far away from us.
When a cloud is directly overhead, I have to look up at it in order to see it, when the same cloud is far away, I have to look straight ahead in order to see it, similarly, they suppose when the sun is directly overhead, I have to look up at it to see it, when the sun is far away, I have to look straight ahead to see it.
The trouble with this is, as far as I can figure, if the sun is approximately 30 miles across, and 3000 miles above a flat earth, like the flat earth wiki says it is, then it should never set, because it's too far from the ground, and/or isn't traveling sufficiently far away from the viewer, even if it travels 3000, 6000, 12000 miles away, it's rays should be hitting your eyes at an angle, markedly, very noticeably, it's rays should never be hitting your eyes from straight ahead, but from up and over, unless it were to travel hundreds of thousands, if not millions of miles away, which are obviously distances surpassing the diameter of the earth.
So there's two possibilities I can think of that are worth thinking of, either the earth is indeed round, or it's flat, but the sun is a lot closer to the ground than the flat earth wiki says it is, well, either that or the earth is greater in diameter, and the sun is traveling a lot further than supposed.
How did I figure this out?
It's easy, just create a scale model of what they're saying, from any point of view standing on a flat earth, the sun's always going to be more/less up, a sun that far from the ground doesn't travel far enough to be blocked by mountains, much less ocean waves, laughs.