You think that’s air you’re breathing now?

Everything you're experiencing is outputted by the greatest virtual reality machine ever invented, your brain. It's all inside your head. None of it is the "real thing." It's just electrical signals corresponding to the forms and interactions around you. Even though we're each living inside our own braintrix, we can use observation, perception, documentation, to make educated guesses and assumptions. One such inference concerns the shape of planet Earth. And there is truly just one way to absolutely know - or get as close as possible to absolutely knowing - the Earth is or is not the shape modern science tells us it is. If the Earth is flat, that way is up, and if it's spherical, that way is out. Seeing is believing, skepticism is the cornerstone of science, and the person convinced Earth is flat is just as foolish as the person convinced it's not.