Bank failures occur when there's not enough gold in the vault and then people start asking for the gold and the goldsmith banker doesn't have it to give out.

当金库没有足够的黄金时,银行就会倒闭,因为当人们开始索要黄金时,金匠银行家却拿不出来

耶鲁公开课 - 金融市场课程节选