The earth is flat, but it is also not, it really depends on where and how you measure or what you compare it to.
Everything depends on how far your eye is to the surface you are observing.
Think of a piece of sandpaper if you stuck it rough side out, against a wall and walked 25 meters away from it, you would be able to see it, but wouldn't be able to tell the grade, it would be too far away to you to be able to discern the individual grains. However if you walked closer the detail would gradually emerge directly corresponding to the distance traveled towards the sandpaper.
To go further: you could also demonstrate this buy using one piece of sandpaper and one piece of flat card that is the same colour as the Sandpaper, stick them to the wall side by side and from 25 meters they will appear very similar in texture, but if you walked up to them you could see the difference between them.
The reason the Earth appears to be flat to us is because we are too close to observe the curvature (unless we are at sea as Ron said).
400 Grit Wet or Dry Sandpaper:
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It is not just distance that effects what we see, angle is just as important: the angle that we view something from can be the difference in the shape we see, for example a Pyramid and Square from certain angles appear to be the same shape, however they are completely different in reality:
The same is true of Spheres and Circles:
If we travel far enough away from planet Earth we can see it is round:
You can see that the Moon is a Sphere with the naked eye (It is round wherever you view it from Earth) and as was known long ago the Earth, Moon and the other planets orbit the Sun and they are all basically spherical in shape.
You can Sail around the Earth (circumnavigation) and you don't fall off the "Edge".
The flat Earth "theory" (if it even deserves to be called a theory) is just fantasy and cannot be supported by any rational thinker.
It is a fantastic mystery to me why anybody would believe the Earth is flat? but I suppose if you want to believe that it is up to you, my best advice would be if the person promoting "the flat Earth theory" is talking about things you are not familiar with, go and look up the information independently, but always ask these questions: if the Earth is flat why can we not fall off the Edge?, where is the Edge? why can we sail around the Globe? Why did and do the greatest thinkers believe that the world is round? why is it impossible to prove that the world is flat, yet it is possible to prove it is round?
Time is precious, use it wisely, there are many more worthy causes for effort and investigation.
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Example A: =
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Example B: =
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