A bank run (also known as a run on the bank) occurs when, in a fractional-reserve banking system (where banks normally only keep a small proportion of their assets as cash), a large number of customers withdraw cash from deposit accounts with a financial institution at the same time because they believe that the financial institution is, or might become, insolvent; and keep the cash or transfer into other assets, such as government bonds, precious metals or stones. When they transfer funds to another institution it may be characterised as a capital flight. As a bank run progresses, it generates its own momentum: as more people withdraw cash, the likelihood of default increases, and triggering further withdrawals. This can destabilize the bank to the point where it runs out of cash and thus faces sudden bankruptcy. To combat a bank run, a bank may limit how much cash each customer may withdraw, suspend withdrawals altogether, or promptly acquire more cash from other banks or from the central bank, besides other measures.