As if their teeth weren't bad enough, even their skin has teeth.
These denticles are thought to improve hydrodynamics by creating little vortexes of turbulence near their leading edges. Whatever their function, shark skin feels abrasive to the touch, like sandpaper. And in an era when no one knew how to make sandpaper, the Japanese found an application for shark skin in the kitchen:
That's a traditional Japanese oroshiki, skinned with shark, used for grating wasabi.
Later the Japanese developed oroshigane, which are graters made of metal.