One of the things you end up spending a lot of time on when trying to write a book aimed at a non-technical audience is coming up with good metaphors for highly technical concepts. This was one of my favorites: my attempt to explain the SMD theorem in terms of blueberries (individual economic agents) being aggregated into pies or smoothies (an economy). Because the agents interact with each other, aggregation has to yield something more complex than "it becomes a pile of blueberries". But we can only really say what some of the properties are (e.g. it'll taste a bit like blueberries) and not others (it'll be blueberry-shaped).