jgrahamc - I think you should introduce the concept of collision here. In the last article when you got to "things", my thought was: there have to be a few words that get to "things". In your reverse lookup table of the dictionary algorithm, there will be a lot of words for, e.g. "the", "or", "and" and so on. This is actually nice - because you start getting to other security concerns from here. For the crossword, you can talk about search spaces: instantly eliminate all words not of length N. For passwords, of course keys with lots of collisions reduce the search space, so that passwords with a high collision probability are less secure. And so on.

I'm not exactly sure what direction you're planning on taking this series, but astute readers, even without a solid grounding in crypto concepts, will notice.