This is the core problem of scientific communication. Flat earthers are an extreme example. The process of arguing it etc can be a good exercise.

Edit: my favourite argument is to look at a half moon, and what angle the "shadow line" on the moon is relative to the horizon. Then ask your friends around the globe to report what angle it looks like to them. Because we are all standing on the side of a globe, we see it at different angles relative to our local horizon, which should perfectly correspond to each person's latitude etc. Fun easy experiment for an online community!