You pick up a seashell, hold it against your ear, and suddenly you can hear the ocean's rhythmic waves.
As a kid, we were probably all very impressed by that, and were told that part of the ocean was trapped inside the shell! !

As you might have guessed, what you can hear there is not actually the ocean.

But how is this sound actually created then?!

There are several popular answers to this question.

Some say that what you hear is the blood flowing through your blood vessels.

But this is in fact not true - test it when you're out of breath and had just exercised. You will notice that the sound is the same, although it should be faster if it really was related to the speed of your blood flow.

Another theory suggests that what you hear is a sound created by air flowing in and out of the shell.

This is supported by the fact that the sound changes, depending on how you hold the shell against your ear - it's louder if you don't hold it all the way against your head, but keep it slightly tilted.
But in a soundproof room, there would be no "ocean sound" inside the shell - although there's still air flowing through.
So this theory is debunked as well.

So what actually creates that "ocean sound" in the shell is the ambient noise!

There is an ambient noise all around us, we just don't notice it normally.
The seashell emphasizes this noise, because it resonates inside it.
A sound from outside enters the shell, and is then bouncing around the inside, amplifying the noise so that you can hear it clearly.
When the shell is small, the air can bounce back and forth quickly, which will make it sound higher pitched than the lower sound of a large shell resonance.
And that's also why the "ocean sound" is different for every shell - the shape and size of it affect the sound that you hear because they accentuate individual frequencies.
You can actually experience the same sound by putting an empty glass close to your ear.