I did one of these from one of my favorite sites, where we were instructed to try to live off what we already had in our fridge, freezer, and pantry. We were not 'allowed' to use the grocery store except for milk, bread, eggs, and butter... and those were only in an emergency. Like a 'cannot-use-anything-else-instead' kind of situation.
In most households, there is a surprising amount of food that is not used to feed us, it is used only as a marker of when items are getting low. For example: I could have 4 cans of green beans on my shelf, but when I get down to 2 (not zero), I get more. Why do we do that? Are we undergoing a famine? Are we preparing for the next y2k? Will we poof into an alternate universe if we finally use that last can of green beans? It is absurd when you think about it. I hear it all the time in the grocery stores as I scan; I am like an invisible stalker, listening to your conversations as you shop. "We only have 12 cans of gravy left, and it's on sale, so grab 6 more, will you honey?" "Yes, Ralph, I know we have enough soup, but it's buy one get 2 free... it would be a waste of money NOT to buy them now." And so on it goes to one extent or another for most of us who find ourselves in the grocery store one or more times per week; not those who purposely plan for months, and yes, sometimes years ahead.
When I took on this challenge, I found that I had to use my noodle about 5% more to figure out what could go with what, but I was rewarded to find I had enough food to last at least 2 weeks, not one. I found buried treasure at the back of my cabinets, I was given a unique opportunity to clean the bottoms of the shelves, got out of the rut of the same old dinner menu, and I learned a thing or two about my shopping habits. It was actually fun!