Cairo is a BIG place. It has a LOT of people as you can see on television.
Its modern, and part of a modern state, relying on imports, transport and a market economy. Farms don't dot the streets of Cairo: food comes from the surrounding areas, or is imported, just like any large metropolis.
To move goods around you don't just need some system to barter; currency in this case. You need communications and you need fuel.
Fuel requires a functional banking system, without a domestic source of oil. Egypt produces no oil. It imports and refines gas. Other Arab oil producers don't take checks and don't take IOUs. Egypt has $36B in foreign cash reserves (per ETX capital on AJ.)
Egypt unrest leads to food shortages
/SNIP
"Everything is running out. I have three children, and I only have enough to feed them for maybe two more days. After that I do not know what we will do," school administrator Gamalat Gadalla said.
/SNIP
"With the curfew, there are no restaurants, food or gas. Basic goods will soon be in shortage," an Egyptian blogger who goes by the moniker Sandmonkey said via Twitter.
UPI
The Unstoppable Force
To an observer it might seem that there is an unstoppable way about the developments in Egypt. Its true that a mass of people this large is potentially unstoppable (although they were prevented from going to the presidential palace.)
My point isn't that the protesters are an unstoppable force, but that their peaceful protest is. Protesters have not, til now, rioted or looted. They are, for the most part, unarmed and are directing their action towards rhetorical protest.
The Immovable Object
People need to eat... roughly 10 times their # weight in calories as a rough guide, to support normal organ function at rest.
Can you survive on less for periods of time? Yes, but any kind of activity becomes more and more difficult. People will find it difficult to protest on foot as they increasingly must, with no food.
If you read the anecdotal accounts, food normalcy is going to end very very soon. In the school admins case by Wednesday or Thursday.
There will be some communal effort to distribute what food there is better, but that is limited by the gas. By the time this becomes obvious to the protesters, gas to move the food supplies around from outside or within the city will be a precious commodity.
The End Game
Hosni Mubarak has shown intransigence, much like the Immovable Object. The protesters on Tuesday have not marched on his palace. They have a lot of energy, expended on anger and rhetoric against Mubarak.
I think Mubarak's plan is to watch the country descend into chaos and then make the case that he's necessary.
You might say that, of course the people won't listen. They want him out. Thats true, but I think that we will see the nature of protest change as hunger grips the protesters.
I think the army could take advantage of this inevitability.