People of Lvov city in Ukraine decided add a really cool art installation to the side of a residential building in hopes of boosting visitorts. According to the project details, a crossword puzzle spanning 100 feet (30 meters) tall at the wall was applied to the sides of a multi-story residential building.
There is one interesting detail about the design of the puzzle. It looks like an empty puzzle during the day, but at night when special black lights turned on, the words in the puzzle become visible with a faint luminescent green-glow.
The questions for this crossword puzzle are located in different point of interests of the city, like monuments, theaters, fountains etc. So people while walking around the city can try to answer the questions and writing down the answers. When the night comes to the city they can meet at this house and check their degree of intelligence. Source