Bond rating agency S&P downgrades the safety rating on the US's bonds. The stock market responds... by selling stocks and buying bonds:

Prices for U.S. government debt rose because Treasurys are still seen as one of the world's few safe havens. The yield on the 10-year Treasury note fell to 2.48 percent from 2.57 percent late Friday. It fell as low as 2.46 percent earlier Monday. A bond's yield drops when its price rises.