What's an Accordion?

So, we're working on ourlapbook, and I'm explaining to Monkey, "This is called an accordion book, because it opens like an accordion." And he's nodding, but it's pretty obvious: he has no idea what I'm talking about. Hurray for the internet! Now he knows! And he gets why it's an accordion book. And why those movable walls at church are called accordion doors. Turns out, Dragon likes the accordion music.