A barnacle is a small sea creature with a hard, cone-shaped shell that permanently cements itself to rocks, ships, piers, and even whales. Once a barnacle attaches, it stays there for life. Barnacles look like tiny white or gray volcanoes clustered together, and if you've ever walked carefully across slippery seaside rocks, you've probably seen thousands of them.

Barnacles start life as free-swimming larvae, drifting through the ocean. But when they find a good spot, they glue themselves down using one of nature's strongest adhesives, something so powerful that scientists study it to create better glues. The barnacle then builds its protective shell and spends the rest of its life in that exact spot, opening its shell when underwater to kick out feathery legs that catch tiny bits of food floating past.

On ships, barnacles create problems. Thousands of them attached to a hull slow the vessel down and waste fuel. Sailors have fought barnacles for centuries, scraping them off or coating ships with special paint. The phrase barnacled hull describes something crusty and worn from a long time at sea.

Surprisingly, barnacles aren't mollusks like clams: they're actually crustaceans, more closely related to crabs and lobsters than to the shells they resemble.