The U.S. Treasury Department generates cash by selling Treasury bills, bonds, and notes to investors. When an investor purchases these securities, they are lending money to the U.S. government in exchange for the principal plus interest at maturity. Treasury bills have short maturities of 3-6 months, while Treasury notes and bonds have longer maturities. The interest rate paid on these low-risk securities varies depending on economic conditions, as seen in the decreasing yields on 3-month Treasury bills from 2007-2011 reflecting the slowing economy.