Question: What would you say to "flat earthers"? How would you prove them wrong?

Dan Porter answered on 11 Mar 2019:

I find the best way to talk to people who have these sorts of beliefs is to try to see their point of view. They can see the horizon and it looks flat, they hear the argument that the world is round but don’t see the evidence and don’t want to look at options other than the popular one. Listen to them, and then simply question the logic:
If the earth is flat, why is there no photographic evidence of the edge of the world? Why arn’t there holiday tours to go there?
If the answer is “it’s a conspiracy” then, given most conspiracy ends up getting leaked online due to incompetence or whistle blowing, and this conspiracy requires every scientist, pilot, ship captain and politician to be involved, how hasn’t it been uncovered yet?

If that doesn’t work, have a look at a video like this: https://www.youtube.com/watch?v=95NDkABAsSk and ask them to explain why the weather balloon at the edge of space sees a curved surface)
NB: There is an issue with these weather balloons showing an more or less curvature because of the curvature of the camera lens, there’s lots more youtube videos to explain this, but they all still show slight curvature – even the ones that claim no to!