You might describe a shark as a toothy torpedo covered in sandpaper:
A shark’s skin is covered in millions of microscopic denticles, rigid tooth-like scales that jut out from the soft skin beneath. By disrupting the flow of water over the fish’s skin, it is thought, the denticles reduce drag, making for a more efficient swimmer. But to really empirically understand how the denticles do their job, you need to see how different sorts of skin coverings affect the fluid dynamics as water washes over the skin of swimming fish. You can’t take a real shark and give it new skin, so Harvard University researchers Li Wen, James C. Weaver, and George V. Lauder created artificial shark skin instead. They manufactured it using a 3D printer.
Lauder’s group then subjected their 3D-printed faux skin to a series of tests in water. They found that it managed to reduce drag by 8.7% when the water flowing over it moved slowly, which is consistent with the thought that denticles reduce drag. But in faster currents, the denticles actually increased drag by 15% compared to a smooth sheet. That might seem surprising at first, but sharks don’t swim in a straight line, they wriggle their bodies. As soon as the researchers started wriggling their artificial skin, the swimming again become more efficient: swimming speed increased by 6.6% and the energy expended was reduced by 5.9%.