What makes that "Clicking" sound from the Clicker?

Hi, I was talking with my girlfriend today and we were sharing ipods, and she asked me an interesting question.

"Ya kno, what realy makes that little clicking sound that you hear when you move the wheel? If its some kind of speaker, don't you think apple could play music through it?"

I was thrown back by the question, becasue its one of those little details that people don't kno much about.

So my question would be, what does make that little clicking sound in the ipod?