There's an impromptu 9/11 memorial on Fifth Avenue this morning using cups, chalk, and good old NYC sidewalk. A sign explaining the installation, which starts at 14th Street and continues for almost ten blocks up the avenue, says that a group called "Illegal Art" has marked each of the 110 floors of the World Trade Center in chalk, heading north for 1,368 feet, the height of the taller of the two towers. "Passersby, like yourself, are encouraged to walk the height of the once standing buildings along 5th Avenue and write any words that express your feelings or experience related to 9/11."

The plastic cups are filled with chalk, and a reader who sent us these photos says the messages on the sidewalk are already growing. It will be interesting to see how this unfolds throughout the day, but some heavy messages have already been left behind.