One of my favourite parts of Prague in the Czech Republic is the John Lennon Wall.
It is a wall that has been decorated in graffiti that purveys messages of love and hope. That and more recent additions by members of Erasmus 2012.
One can spend great lengths of time studying different parts of the wall. And every time you go back, you know that it will be different. It is changing organically because of the people who choose to contribute to it.