Somehow I did it the first time, but the attached file is giving me trouble. Very annoying. Cannot make the thin extrusion thing go away consistently. I "attached" (anchor icon) the second extrusion's sketch, and it seemed to work, because it was magically solid. I am trying to do the same with the third sketch, I "attached" them but it just won't let me NOT make a thin extrusion.
So what do I need to know?