This is a functional knife made out of katsuobushi (dried,fermented and repeatedly smoked skipjack tuna). Katsuobushi holds the record for being the hardest food on earth, even though I've been eating rocks since I was a kid, and no problem. Katsuobushi is typically shaved from a large piece with a special wooden box plane for use in soup stock or as a food garnish. Apparently it can also be repeatedly shaved and filed into a function knife. So,the next time you're stranded on a abandoned island with nothing but a piece of katsuobushi, you can relax knowing that the shark you eventually determine to feed yourself to will get some additional nourishment from the piece of dried fish in your pocket, or that's a comforting thought.
withhold going for the video.
Source: geekologie.com