Have you looked out your window recently?
Clear precedent for a flat earth.
In the vein of rational discussion:
Let's suspend disbelief a moment and pretend the Earth is a sphere. Do you believe that, upon looking out the window, you would visibly see the curvature of the earth? Do you think every road would appear to be a hill, and every horizon a curve?
You do know that if the earth were a sphere, it would be such a large sphere that it would be imposible to notice such a curvature from the ground, right?