Explain xkcd

It seems I was nearer finishing than I thought.  I've tested this with spot checks and I can't see any show-stopping issues.  Not sure I've got the Core Data stuff working properly as I'm quite new to Core Data.  It looks like the explanation for each comic isn't persisting properly, but perhaps you know more and could get it persisting properly.  If there are changes that you want to make, can you let me know what they were so I can learn from it?  Cheers!