You will likely never need to make a cutting edge out of discarded cigarette filters, but if there's one thing you can find just about everywhere, it's cigarette butts, and if you do ever need the know-how, this video will show you so you can survive -- or just show off a fun MacGyver trick for friends. Make no mistake, if you are ever stranded somewhere without a blade (which, if you're carrying a good multitool, you shouldn't be) the odds of you finding cigarette butts with the filters still in good enough condition to make one of these is definitely higher than you finding something sharp enough to cut with. You will, however, still need a lighter or some other small flame and a rock, as shown in this video by YouTuber Russian Tony (who also points out that this is, as you may have predicted, a prison trick).
Heat the edges of your cigarette filter until they're soft enough to mould, but sticky enough to hold their shape. You'll want to use two or three filters for this, but once they're warm and soft, press them down together as flat as you can until they're thin, but strong. Then hone them against a smooth rock until you have the edge you want. You can actually get this surprisingly thin, as Tony demonstrates on a hot dog near the end of the video.
It's not going to be the most effective blade, especially if you need to cut through something thick or defend yourself, but if you need an edge for something else, or you're in a pinch, it's worth remembering.
DIY: Blade From Cigarette Filter (Survival Life Hack) [Russian Tony (YouTube)]