The central bank can influence the economy by changing the amount of money and other assets available. Different assets provide different levels of liquidity, with money being the most liquid. When the central bank adjusts the supply of assets, it can impact overall liquidity and welfare. In situations with low inflation rates, the central bank's asset purchase programs may not be effective. A liquidity trap can occur when the real interest rate is positive, away from the Friedman rule. Additionally, restricting trade with government bonds can affect welfare.