ELI5: How do we know the world isn’t actually flat?

My 6 year old cousin has become convinced that the world is flat. I told him that the reason I know the world is round is because the people in the Louvre have posted pictures of da Vinci’s globe, but he told me that could easily be faked. Then I tried to tell him that we know the world is round because ships disappear from the bottom up when they move away, but he told me that the atmosphere has actually been proven to be very dense, so if the world is flat it would make sense that we wouldn’t be able to see the entire ship. I didn’t have a strong argument for either of those points so he told me to prove it, and I don’t want to disappoint him. So, please. For the sake of my poor, misguided cousin. Explain to me like I’m 5 why the world isn’t flat.<br><br>ETA: I should probably add that I understand he’s wrong and so do most flat earthers deep down. And I also know that there is a difference between being wrong, and being mean-spirited. This is a friendly and well-intentioned six year old. I’d never call anyone stupid just for believing things that aren’t true. I do this because I love him, not because I’m going to mock him for his beliefs or call him stupid. Thanks for all the help.<br><br>As for everyone saying they can’t believe I didn’t have a strong enough argument for a six year old, I don’t expect anyone to have all the answers for everything all the time. There is literally no human alive who knows everything, and to act like it’s not okay to ask for help is honestly a bit absurd. This was literally made to help people understand things more easily, so I asked for help. And I appreciate it.