When you look at water in a cup, it looks flat -- I completely agree. But imagine for a moment that we live on a globe earth: how could you be completely sure that what looks flat in the cup isn't actually very very slightly curved?

If you zoom in too close to a segment of a circle, eventually your eyes aren't sensitive enough to see that it's curved and it will just look like a straight line. How do you know that's not what's happening in the cup?

By the way, if you ever get frustrated by my answers and you want to stop talking, I won't be offended in the slightest bit if you just stop replying to me. I'm not here to make anyone angry, I'm just trying to promote good critical thinking.