I know you're joking, but this is highly useful in this regard, because flat-earthers often swear up & down that they can see beyond the "ostensible" horizon limit without understanding that refraction effect. This one is kind of unique but it demonstrates refraction nicely.

Really, I think we're usually so busy being dismissive and condescending towards flat-earthers, it's easy to forget that proving the spherical earth is kind of a tricky matter without putting people in spaceships (but if I could afford it, I'd happily buy a few tickets for them...)