Shells are like ears.

A conversation between me and my youngest sister (she's blonde but brilliant):

Me:  Did you know shells are like ears?

Blonde but Brilliant:  No.

Me:  Now you do.

BBB: … Okey.

Let me try to be more informative.

I remember once when I was small, I was listening to the sound of the ocean in a conch shell with my cousin.  She smiled at me and said she thought the ocean sound would stay in that shell for a long time.  That was when I began to wonder why the shell sounded like the ocean.  Was it recorded, like my cousin said, and could it run out?  I strained my ears to try to see if I could hear seagulls.

There was a shell in my kindergarten classroom, and I used to listen to it all the time.  The boys in my class said it sounded more like a toilet flushing.  I secretly agreed.  And that made sense with my cousin's theory, because there was a bathroom near our classroom, and not an ocean.

Later, in second or third grade, I was told that the sound of the ocean is caused by the echo of the blood pumping through your ears.  Rather than feeling disappointed that I wasn't hearing the real ocean, I smiled when I heard that.  My blood sounded like the ocean.

At home, I liked to listen to that shell my cousins and I had found on the beach and think of the ocean flowing through my veins.

Now I know that the shell is actually a resonant chamber—it emphasizes certain frequencies with the noise around you and creates the ocean-like sound.  Our speech and hearing systems include several resonant chambers—namely the vocal tract (the space between the larynx and the lips) and the outer ear.  Like a shell, the ear doesn't change shape.  The ear acts as a resonant chamber:  as sound waves pass through the outer ear and ear canal, these structures vibrate with the sound and accentuate certain frequencies.  Conveniently, frequencies important to understanding speech are emphasized in our ears.  (Cause and effect are fuzzy here—which developed first:  speech frequency contrasts or ears?  Or was it simultaneous/complex?)  Apparently, the world agrees that shells resonate with frequencies that sound a lot like the ocean.

That means that everywhere there is sound, the sound of the ocean can also be there.  You just need a good resonator, like a shell.

It also means that shells are like ears.