Introducing Modal Operators to Intro Students

I find it pretty much impossible to teach philosophy at any level without using and mentioning modality. I also have terrible handwriting and am lazy, so symbols are really handy. Since I mostly teach intro type stuff I can't go crazy on the symbols, but this year I'm once again using a strategy I've found to work in the past and thought I'd pass it on.

I introduce possible worlds by talking about stories about the way things could have been--which, a la Adams, is in fact how I tend think of them--then I tell them that I'm going to use a box with an 'N' in it to stand for necessities things that must be true in any story and I write [N] in front of some necessary statements to make the point. Then I introduce a diamond with a 'P' in it--

--to stand for statements which could be part of some story. Gradually throughout the class the 'N''s and 'P''s get smaller and smaller until the disappear. By the end of the semester the box and the diamond are part of their vocab and they automatically understand them. The strategy generalizes more or less to other symbols as well.