What is the primary tool that central banks use to increase the money supply during an expansionary monetary policy?

The primary tool that central banks use to increase the money supply during an expansionary monetary policy is open market operations. This means that the central bank buys government securities, like bonds, from banks and financial institutions. When the central bank buys these securities, it gives banks more money, which they can then lend out to businesses and individuals. For example, if a central bank buys one million dollars' worth of bonds, the banks receive that money and are able to offer more loans, encouraging spending and investment in the economy. This process helps to stimulate economic growth by making it easier for people and businesses to access the funds they need.