I've seen some very nicely formatted Mathematica notebooks with sections, explanatory text, etc., but I can't figure out how to do something similar. The documentation is not especially helpful here so I'm asking here.
I see you can group cells together, and then I've tried putting a line at the top to be a section header - say, "Preliminaries." In this cell I've done a few different things to try to make it text - Cell/Convert To/Text Display, Cell/Cell Properties/Uncheck Editable, Format/Style/Section - but still the cell saying Preliminaries is evaluatable. Doing practically anything - even right-clicking the text in that cell - will change its format/style back form Section to Input.
How do I make a true section header, a cell that really contains text and not something that Mathematica can evaluate?