Subject: Why does boiling stock with egg whites and egg shell clarify it?

I was reading a recipe book while making soup today.  It said that to clarify
stock, one could boil it for 2minutes with egg whites (lightly beaten) and
crushed egg shells.  One white and shell per litre of stock.  Then let it sit
for 20 minutes and run through cheese cloth to strain it.

As my stock wasn't too clear, I tried it and it worked.  Now I'm wondering what
kind of neat chemistry trick makes this work.

Re: Why does boiling stock with egg whites and egg shell clarify it?