The Earth is not flat 

When you are on a ship as it leaves the port, you will see that it eventually disappears since the earth is curved.
If you see the shadow in one city and compare the shadow in another, it would be a different length and orientation.
Celestial objects you see, like the moon, sun, and planets, are all round.
Flying on a plane, you will see the curvature of the earth from a high elevation. Even people who sky jump can see the curvature.
Even with all this evidence, some people still believe the earth is flat.