"Be not so bigoted to any custom as to worship it at the expense of truth."
Johann Georg von Zimmermann

...And All The Ships At Sea

The strongest argument in favor of a spherical Earth is the phenomenon observed when ships sail out to sea. As ships reach the "horizon" they apparently seem to sink so that the masts are the last thing seen before the ship is fully out of sight. This phenomenon puzzled the flat Earth believers for centuries until it was finally explained (somewhat) by Einstein. It's a simple explanation of gravity bending light. This light bending also explains why half the world experiences night if the Sun never sets. It's a bit hard to explain with words so bear with me. There are pictures below which will help explain.
The best analogy can be made with cannon balls. When a ship is near the shore a cannon ball fired from the deck will easily reach the shore. As the ship moves farther away, gravity causes the cannonballs to fall short of the shore. The only way to get the cannon balls to reach the shore is to mount the cannons higher up the mast.

As you can see in this image, when the ship is close to shore, all light can reach the observer.

Farther away from the shore, the light from the lower parts of the ship are bent by gravity so that only the masts are viewable from the observer.

As the ship moves farther away still, the light from the lower points become bent so that almost the entire mast has disappeared beyond the horizon.

Of course these images are not drawn to scale (and please, no comments about my artistic skills), but they do show that gravity bending light can cause the same phenomenon that Sci-onists attribute to a spherical Earth.

"Facts are facts and will not disappear on account of your likes."
Jawaharlal Nehru