@Jacques6 - I was planning on build a bookshelf, so I checked out this article. I know that a lot of parents let their kids make the mistakes to teach them how to learn on their own, but wood working is a hard thing to undo if there's an error. I would definitely explain that to my kid before they started building. It sounds like your mom helped you fix it, which was nice.
I was designing my bookshelf to be two and a half feet tall exactly so that it would fit under a table -- it's good to know that I'll have to make it a little taller. I had no idea that the wood shrank. I'll have to buy another board now.