Cardboard boxes are the workhorses of the commercial shipping industry. Just about every consumer product you can imagine — shoes, cans of soup, tennis balls, light bulbs — is shipped in "corrugated" as it's known in the industry. That's why the manufacturing and sales of cardboard boxes are such reliable and accurate indicators of economic performance.
Here's how it works. Consumers have more money to spend, so they buy more socks. The store notices that it's about to run out of socks, so it orders more from the factory. The sock factory, noticing an uptick in orders, goes out and buys more cardboard boxes to ship out those beautiful socks. The nice thing about cardboard is that it's not specific to socks or any single industry. An increase in cardboard orders signals a widespread increase in consumer activity [source: Obel].