Snails, turtles, hermit crabs: all living things with shells. But an empty shell sometimes appears to be alive too — like when you hold it up to your ear and hear the echo of the ocean. It’s an illusion, of course: the shell is nothing more than an acoustic filter, transforming the sounds around you into a mimicry of waves crashing on a beach. Reality is not always what it seems. After all, that shiny, new shell you picked up on the beach? Chances are that it’s a million-year-old fossil.