If interest rate reduces below the equilibrium level, there will be excess demand for money. Excessive demand means that economic units wish to hold an amount of money more than the money supply of central bank. As long as money supply does not change, excess money demand will cause a pressure on interest rate to rise. In an environment where money supply does not change, economic units will try to have the additional amount of money they need by converting some of their financial assets to cash. One of these assets is bills they hold. When economic units start selling bills, the price of these asset decreases and therefore their interest rate increases.