The article explains how subprime mortgage loans are bundled together and sold as investments. It identifies seven challenges in this process and discusses how they were managed, but also how things went wrong. It talks about the risks for both borrowers and lenders in these transactions. The article also describes how credit ratings are assigned to these investments and how their performance is monitored over time. The researchers use the example of a mortgage pool from New Century Financial in 2006 to illustrate these points.