Sharks have often been described as swimming noses, a claim that has considerable support. The nostrils, which are located on the underside of the snout, open into olfactory sacs that are lined with many folds of tissue containing numerous chemoreceptors. Water enters the nostrils either by the suction force created when a shark takes in water to aerate its gills or by the forward motion of the shark as it swims. Ingoing water is separated from the outgoing water by a fleshy flap, which divides the nostril into two passages. The acuteness of a shark's sense of smell has been shown in experiments where some sharks respond to chemicals in concentrations as low as 1 parts in 10 billion.