You can't - what you are hearing is the balance fluid and blood circulation which issues a 'rushing' sound, which is then reflected back from the shell, and picked up by your hearing. Because it sounds like the sea breaking on the shore, generations of parents have told generations of children that they can 'hear the sea' inside a shell. Sadly, like a lot of things our parents told us, it's not true. Next week, The Tooth Fairy, and where those baby teeth really went to during the night ...