system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: With Greek closing its banks on Monday in fear of a bank runs, what is a bank run, and why do they have the potential to do so much damage?
gpt
It's when everyone decides to withdraw all their money at the same time.

It can bankrupt the bank (make it completely run out of money), because they have loaned out some of the money for mortgages, business loans, etc.