Used books and their previous lives.
I started reading a book today that came in a lot Matt bought on Ebay. I noticed a little pink piece of paper marking a page only about a fifth of the way into the book, and I started thinking about it way too much. My first thought is that the person reading this before me decided it wasn't worth finishing. So I started to wonder if it's just not worth finishing. Then I really got into the other possible scenarios including death or simply sticking that paper into a random part of the book while they finished reading it and then forgetting it was there. I'm pretty sure it's not normal to think this deeply about something so trivial as a scrap of paper.