One way to reduce energy usage and to make our urban environments more sustainable is to construct our buildings with more energy-efficent materials. This includes better insulation for our homes and using new types of glass to regulate the amount of light allowed into buildings based upon the season of the year. For the construction of the swimming stadium for the Beijing Olympics, this same concept of regulating the amount of sunlight allowed into the building was used - but instead of glass, the exterior of the building was constructed of a special plastic "bubble-wrap" that could be electronically controlled so as to allow or restrict light into the building.