People put their money in bank accounts. The bank uses that money to make loans or purchase investments. Banks make a profit by lending out your money at a higher rate, for example they can pay 3% for your deposits, and then lend it to customers at 7%. Banks have to manage their liquidity to ensure they have funds on hand for when customers withdraw their money. If they don't, the bank could fail. That's why bank accounts have deposit insurance up to $250k

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