The main channel for banks to make money is lending, but many people in the United States can't buy a house because they have a poor reputation in the bank and the bank won't lend them money. But when the property is good, the bank thinks that the property is so good that it can be loaned to these people. In fact, it is relatively safe, because even if these people don't repay their loans, banks can still auction their houses to make up the bills, so many banks lend to these people. But the more bank loans, the less money he can invest, so he packaged these loans into bonds, which are a kind of high-interest bonds in the form of transactions (the interest received by the buyer is the interest received by the bank from these homeowners). These bonds look delicious, so they have become the portfolios of many funds and investment banks. Banks rely on "Fannie Mae and Freddie Mac" (taken over in February) to issue bonds in the bond market, and then lend and issue bonds when they have money. This cycle can basically be counted as a loan of 16 days, which can be packaged into bonds.