How to easily explain the world is round?

Ok, so I got into a debate with someone at work, who was debating how do humans actually know the world is round. He discredited satellite imagery and basic common science. I had a hard time finding an actual ‘bulletproof’ example of how the world is round. My theory was that if you leave in a plane from your current location, pick a direction, and continue on that course…you’ll eventually come right back to the same place you started. Hence why the world is round…he wasn’t buying that. Is there an easy way to explain why the world is round…like a great simple solid example I can use? Thanks

Edit: These are some great examples and points. Keep them coming! I have trouble articulating things sometimes during a ‘debate’ in person so all of this helps