About 30 kilometers from Denmark's capital of Copenhagen, lies a small, but significant district called Musicon. Sit on a bench in Musicon, and you'll likely be sitting on slabs of concrete salvaged and repurposed from a demolition site nearby. Or bring your kids to the skatepark, and they'll be riding their scooters on concrete that used to be a basin and canals for collection of rainwater. Musicon was founded in 2007 on the premise that the old concrete factory that occupied the site should ... become the foundation for the new district's development. This meant that new construction projects would have to reimagine the old factory buildings in creative ways to create structures for living and working. This is one example of what is called a circular economy. To become fully circular means to avoid as much waste as possible, and to preserve as much value in what does go to waste. City planners have been cozying up to the idea of circularity in recent years, typically with the hope of combating climate change and resource scarcity, and many have begun embracing the approach. The CityLoops experiment ... aims to create sustainable city planning solutions based on the premise of circular economy. In several participant cities, including in Musicon, the circular economy takes the form of "banks" or "marketplaces," digital and physical, where salvaged materials are stored and offered up for use in other projects in the area, including anything from a birdhouse to an apartment block.