Sharks are famous for their sandpaper skin. Its roughness is caused by thousands of microstructures called dermal denticles ("tiny skin teeth"). To call them bumps is to miss the elegance of their forms, for they are shaped like so many ribbed delta-wings and overlap in a precise and beautiful geometry. These denticles maintain a smooth flow of water by creating micro-vortices at each placoid scale. The micro-votices are a turbulence management system, if you will, and obviate the creation of large swirls of liquid against the shark's skin by creating many smaller swirls. Less turbulence means less drag, and therefore faster and quieter movement.