Whenever I walk out onto 1st Ave, or am in the playground with my kids in front of our building, I look up and see this little tree that someone has planted on the roof of the building across the street. I know a few people in that building and I asked them about it - "Hey, what's with the tree on the roof?" No one knew anything about it, or even knew it was there. So I got my friend in the building to give me access to the roof to show him.
And there it was, the little tree on the edge. (my building is there on the left, top..and you can see the playground there too)