Something that will without a doubt be very important to you post TEOTWAWKI is a knife, and a sharp one at that. While having a knife seems second nature, having one that is sharp and keeping it that way can pose some challenges. You may be forced to use your knife more than usual, which will dull the blade faster than it might have dulled under the demands of normal use. While sharpening stones or other handheld sharpening tools are ideal, you have to have them in order to be able to use them.
If you've ever tried to cut with a dull knife, you already know how frustrating it is. Something as simple as peeling potatoes can frustrate you intensely, but it can get even worse than that. If you hunt and have ever tried to skin a deer with a dull knife, I am willing to bet you got nowhere fast. On my first hunt, back in the days when I did not know any better, I did not have a good, sharp knife. The knife I had was literally laughed off by the deer hide I was trying to cut through. Cutting animal hide is way tougher than cutting your own, so even if a knife seems sharp enough to cut you, that does not mean it is sharp enough to skin an animal by any stretch.
Part of survival could entail cutting tree limbs and ropes with the kind of frequency that can dull a knife. If you want to survive, those tasks will be part of the inevitable. You also have to eat to survive, so your knife is going to have to be ready at all times to deliver on multiple fronts. Whether you are skinning cucumbers or squirrels, you need a properly functioning instrument.
To keep your knife sharp enough to stand up to these tasks, manual sharpening is necessary. Ideally you will have access to tools for sharpening, but if not, you may have to make do with something else, such as a coffee cup, plate, or bowl. Surprising though it may be, both of these objects can serve as knife sharpeners in a pinch. They will not make your blade as good as new, but it will be much improved, which could be all it takes to get you through.
When you turn your coffee cup, bowl, or plate upside down, you will see an unglazed ring on the bottom. When you run your fingers over that ring, it will feel coarse to the touch. Take your knife and run the cutting edge over it, alternating from side to side. This coarse edge combined with that repetitive moment will sharpen your knife and restore it to its functional days. Repeat as needed to keep your knife in good working order so it, and you, will hold up to the demands survival may be thrust upon you.