I vividly remember reading outside of Subway one day when some hippie-looking guy came up to me and asked me what I was reading. Somehow, we got on the topic of planets. This guy, with his faded green polo shirt, khaki shorts and shoulder-length hair, says something along the lines of, “I don’t know about you, man, but there’s this video called ‘200 reasons why the Earth isn’t a spinning ball…’” I knew the Earth was round of course, but I didn’t know how to prove it to the guy. So, I spent the next two hours listening to his rant about a flat-Earth conspiracy among many other, equally delusional topics. That’s why I’m going to arm you with knowledge here. Rest assured that, after reading this, the next time a hippie-looking guy approaches you outside Subway and mentions a flat Earth, you can easily prove him wrong and avoid the inbound flat Earth sermon.

Go skydiving 

One great way to convince your fellow flat-Earther that the Earth is indeed round is by simply going skydiving with them. Make sure you’re skydiving from a very high distance, though. If you’re high enough, the globe shape of the Earth becomes greatly accentuated. It’s so blatantly obvious that the Earth is round at that point that you’d basically have to be crazy to deny it. Now, you’re probably considering just taking them on a plane instead. That could work, but keep in mind the delusion has sunk its roots so deep in the minds of these people that they may literally just blame it on the “curvature” of the plane window. It’s happened before.

Watch a ship sail away until you can’t see it anymore

…And try to calm them down when they think it’s sinking. When you watch a big ship sail off into the distance from a pier or something, you’ll notice that when the boat gets far enough away, the water between you and the boat will obscure the bottom half of the boat, because it follows the curvature of the Earth. From afar, it’ll look like the boat is sinking, which simply could not happen on a flat Earth. 

Take them to see a lunar eclipse

Though flat-Earthers actually have disproved themselves right in front of the entire world, it’s still not enough to convince most of them that we’re living on a spheroid. This is probably one of the most undeniable forms of evidence on this list. Take them out to a lunar eclipse, and watch their mouth go slack as they see part of the giant, circular shadow that the Earth slaps on the moon. Make sure you really make it clear to them that the shadow is not, in fact, the shape of a hot dog. Seriously, they can’t come up with any excuses after that. What are they going to do, say the government has a spotlight in space or something? Actually, I better watch myself. They might get some ideas.

Fly them from Los Angeles to Sydney

Yep. Take them on a nice, long flight. Not for them to see the curvature of the horizon from up in the sky, oh no. When you fly them from LA to Sydney, take careful note at how they sleep in the hotel after the flight. You’ll most likely notice that they’ll have some trouble. Now, take the liberty to explain to them what jet lag is, and more specifically, educate them on time zones. One thing that should click in their heads is that time zones and jet lag cannot exist on a flat Earth, because the sun would be hitting the entire planet at once. 

Take them literally anywhere they can see the horizon

This last one is brutally simple. Just take them somewhere where the road seems to stretch out infinitely or to the top of a mountain or something. Just as long as the horizon line is visible from every direction, you’re good to go. Now, ask them to point out the Sears Tower. Mt. Everest. The coast of Australia. The Himalayas. Of course, no matter where they are, they won’t actually be able to do this. This is because the Earth is round, making the horizon line impossible to see past.