As an American, my knowledge of London is pretty limited -- you know, big city, Big Ben, big Olympics, yeah, I get it. But it came as quite a surprise to learn that there is another London inside of London.
In this video, serial mindblower C.G.P. Grey explains how the City of London is a very special micro-city entirely contained within the city that is called London. The larger city called London has a Mayor...but the City of London has The Right, Honourable, The Lord Mayor of London. Each enforces separate sets of laws, and collects separate taxes. It's deliciously confusing -- even Wikipedia's City of London page says: "Not to be confused with London."
How did this happen? Grey's short answer: because Romans. Get the full story: