When you look at a live fish, you can't see its ears because they're entirely internal, located inside the head just behind each eye. Each ear is a small, hollow chamber, lined with nerve hairs and containing three otoliths — or ear stones — that rest lightly on the nerve hairs. Here's how otoliths help fish detect sound. Sound is conducted through water as a series of pressure waves, which cause the whole body of the fish to vibrate. The ear stones are much more dense than the rest of the fish, so as the chambers of the ear vibrate, the stones lag behind, bumping and stimulating the nerve hairs. The nerve hairs then send a message to the brain that there’s a ruckus going on. In small fish, the ear stones may be just the size of a tiny grain of sand. Large fish have ear stones as big as marbles. And the ear stones actually grow along with the fish — every day fish produce small amounts of calcium carbonate to be layered onto their ear stones. In fact, examining ear stones is one of the ways biologists determine the age of fish. And biologists aren’t the only ones interested in ear stones — Native Americans around the Great Lakes once collected and valued the otoliths of large fish.