I find these people so fascinating. Imagine the global conspiracy of pilots, aerospace engineers etc. I wonder if they've just never flown before.

I don't want to prove to them the earth is not flat. I'd rather hear them explain why its flat, both to satisfy my curiosity and maybe by asking enough questions they realize it isn't flat ... or maybe they convince me it is flat lol. Either way win-win.

You can't really "win" a debate from first principles against someone who has gone so far down a rabbit hole that they've memorised every comeback argument in advance.

The online flat earthers have spent decades prepping all their replies to the common rebuttals and have entirely made their minds up by the time a discussion begins.

Ask them how Hiroshima & Nagasaki happened? Those planes with very very limited range fly all the way down the bottom of Argentina, South Africa over the top of Australia and up the side of Indonesia? Considering we've only just been able to get planes able to do London to Darwin. Hell of a feat to take the route flat earthers think they must have done.

The best argument against flat Earth is simply your shoes...

Your shoes, if are bald at bottom - are bald at the beginning and at the end of the shoe. There is no better proof that the earth is neither convex nor flat, but: concave. ;)

Take them to the beach and show them the ship that is fat away on the high sea... why it is appearing first from the top then gradually the full length is appearing. It is because the earth is spherical.

Try to point to a christian that bible has many flaws. It's just same thing with flat earthers, alien guys, or other stupid superstitions: as long as they "believe" unconditionally they wouldn't change their minds until they start to not trust, but verify. Or until reality punches them real hard.

Give a kick in his butt and wait until he returns from the opposite side. Don't discuss with idiots.