A bank lends out way more cash than it can pay back out to its depositors. Shock, horror...but it works most of the time and enables all sorts of ventures.
As a mother I offer 24/7 medical, emotional and practical care. However, were both my children to actually need this for more than a certain amount of time I would go broke. Same with firemen, the lifeboat crews, accident and emergency, politicians too. We rely on the fact that the service we offer is not asked for that often and there is time to recover or replenish our reserves in preparation for the next crisis.
I enjoy seeing parallels in life, even if they are a little bit rough and ready.
http://www.channel4.com/news/no-reason-to-pack-up-camp-say-st-pauls-protesters This is a short article about Hector Sants and the Occupy discussions.