I have got a problem with a model that gets messed up when I add a subdivision surface modifier to it. Until now I have just redone the modelling to avoid this kind problem when it has occured, but this time I figured it was time to ask here and learn how to fix it properly instead.
In this case I have a very basic shape which I have extruded and subdivided, but the result isn’t smooth at all:
What is the problem here? How can I fix it without having to redo everything? I tried recalculating normals but that didn’t help.
.blend file: http://www.pasteall.org/blend/18491