If you live in a major metropolitan city, you've perhaps noticed that the sidewalks are covered with black dots the size of quarters. I remember it took me a few months after moving to New York City to figure out that was gum. After it's spit out, it gets stepped on and flattened to the concrete. In no time at all it accumulates the ebony patina of city grime. Sprinkling the sidewalks is easy. Removing the tarred, synthetic rubber is the hard part. If you're talking about one chewed stick in a school hallway, it's a can of solvent and a putty knife away from coming off the floor. But when you've got miles of city walkways to clean, you need something better.