Over 2000 years ago Erastothenes showed that the Earth is round by measuring the length of shadows at different parts of the world. He even used the measurements to estimate the size of the planet. A person living in the US could repeat the experiment for the cost of travel, well under a thousand dollars. It requires only the knowledge of trigonometry, which is taught in high school. I find the fact that people believe in flat earth interesting. They've been lied to by the government so much that they have become extreme skeptics of the official narrative, which is a good thing. But they've gone too far and don't treat alternative theories like flat earth with the same skepticism. They accept the alternative even if it doesn't fit the facts. I like the topic because it's linked to epistemology: how do we know what we know and what does it mean for one theory to explain the facts better than another. This is still an area in science which has not been settled. It's called the demarcation problem: How do you distinguish actual science from crackpottery?