Here is logical proven way to show Earth can't be flat. Sun goes bellow the horizon, so if Earth was a flat plane, it would not be possible to have daylight anywhere on it at that time. This is the most simple way to show Flat Earth idea is just plain wrong. Also, when there is no acceleration (due to gravity) water naturally wants to form a sphere, which implies that when all objects in space while they were being formed formed into a sphere... and here is another clue... why would Earth be the only one that is flat, while we can see that other planets and sun itself and moon, are a sphere?