Which is pretty cool in and of itself; I mean, you have this big white wall, right? Why not do something cool with it?
But wait, there's more:
Questions for the puzzle can be found in various locations around the city, on monuments, theaters, fountains, etc. Each evening people meet at the building to check their answers. The way this works is that in the day time the puzzle is empty. But at night special florescent lights come on, and the answers in the puzzle become visible.
That. Is. Awesome.