Probably because of how young I was when I first read it, it took me an embarrassingly long time to notice the plot relevance of the pictures Polly helps Tom pick out in Fire and Hemlock.

I also didn't get the onion thing for the longest time. In the movie Holes, they just start chowing down on onions and smiling, and even though I think they say the onions are sweet, it made no sense to me. I knew onions came in different colors, but since I didn't start cooking until a few years ago, I paid zero attention before then to the difference in taste.