You know you've grown used to Beijing when you can fend off the art students with no effort.

Also, I had a conversation with a friendly People's Armed Policeman today.  He was holding a machine gun.  On one hand, it was a little unnverving.  On the other hand, it was exhilarating.  He was very friendly.

DeadChingis wrote a good summary of the different <a href="http://thorntree.lonelyplanet.com/thorn/repliesflat.pl?Topic=700196&amp;view=expanded">uniformed people you'll see in Beijing</a>.