Do you remember what you asked as child, when you realized that the world is a sphere? "How can the people at the other side of the world walk upside down?"
In SL you can check how it feels to walk upside down. in Citysphere it is possible with a walking animation. you can click the street anywhere and jump on the cube that appears.
But this walking upside down made me dizzy and loosing orientation. Sorry, that the photos went a bit unsharp ;)