The easiest way to prove the Earth isn't flat

Sometimes, it takes a herculean effort to deny reality, almost to a point where the lack of brain power being used becomes impressive unto itself. Such is the case with flat-Earthers, who insist that the earth is shaped like a Parcheesi board rather than the sensual sphere that science has shown it to be. If you are so unfortunate as to become embroiled in a debate with such a person, here are a few quick and easy ways to prove that they need to pull a Billy Madison and go back to grade school. 

PopSci says it can be as simple as watching a ship leave harbor, or come into port. If you do so, you will see that after a certain point, the ship will dip out of sight, or seemingly emerge from the waves, depending which direction it's headed. This of course is due to the curvature of the earth, and not because it's a demon ship from Pirates of the Caribbean.

You can also try riding a plane, which should allow you to be able to see the curvature of the earth with a high enough altitude, but in theory, you could also just ride the plane continuously until you've circled the Earth, which would be impossible if it were square. Planes are also a great way to witness the "magic" of time zones, which wouldn't exist on a flat Earth, because the sun's rays would be able to hit everything at the same time. This same logic could be applied with a simple experiment in which two people on different parts of the earth measure shadows. A flat Earth would provide the same length for each shadow measured, but this isn't going to happen, as the shadow lengths will vary based on their relative position to the sun.

Lastly, lunar eclipses also do the trick, as they give us a chance to see the earth's shadow on the moon. According to NASA, even Aristotle, who lived well before the Ask Jeeves era, noticed that the shadow was round, rather than the thin line we would see if the earth were indeed shaped like a pizza box.