Why do I get a "seam" using Mirror modifier? I started using Blender a few weeks ago and right now I'm starting my first project without a tutorial. I'm starting to create a character and the first thing that came to my mind was "It needs to have symmetry", so I used a Mirror modifier on my object and it has this "seam". In the image I used a Sphere because it is much more noticeable.

So, how can I fix it? And why does this happens?
You should turn on clipping, as Cebbi has already mentioned. Also, the subsurf modifier should go after the mirror modifier. 

That should do it.