Few creatures are more frightening than sharks, but these complex fish have also provided inspiration for several useful technologies. Throughout history, shark skin has been used by many cultures, as sandpaper, non-skid shoes, and the like. Swimsuits modeled on shark skin are claimed to reduce drag by up to 4 percent. Now, synthetic shark skin could make ships and submarines faster and less expensive to operate. The growth of barnacles, mussels, and algae increases drag by up to 15 percent, and adds to fuel costs for commercial shipping and the military. Paints laced with copper curb the problem, but must be reapplied periodically to be effective. They are expensive, and toxic to other marine life. Fish, crabs, and whales are fouled by hitchhiking marine life, but rarely sharks. Scientists have discovered that part of the secret is in the complex design of their scales. Shark scales are made of a hard material called dentin. Like tiny teeth that all point backward, they make a shark feels smooth from head to tail, but rough the other way. The scales flex individually, limiting the amount of exposed surface area on which organisms can attach. They also create tiny vortices that reduce drag, making the shark faster and more energy efficient. Barnacles make some of the strongest adhesives known, but the glue can only penetrate so far into a rough surface, explaining why scales can prevent them from sticking. With the fake shark skin applied, a ship moving at 4-10 knots becomes self-cleaning, leaving most fouling organisms in its wake.