This flexible and reusable building prototype originally came out as our reaction to the unique Chinese phenomenon in recent decades’ building frenzy -- the production of vast amount of flamboyant but short-lived showroom buildings for the sales of real estate developments. Yet it is a building system that can be easily adapted to many different functions, and most importantly disassembled after each use and reassembled in another location, thus preventing huge waste of resources. It is part of our continued efforts in exploring the ultimate potential of building sustainability - reusable buildings. Enabling speedy construction, it is a building system that is light, industrialized, flexible, sustainable and reusable. The prototype V1.0 was realized in Beijing China in 2013, and the prototype V2.0 named HEX-SYS was completed in Guangzhou China recently in 2015. This prototype was inspired by the ancient Chinese wooden building system which can be taken apart and rebuilt elsewhere with almost no damage. V2.0 is more suitable for the climate of south China. We designed this prototype comprised of hexagonal cells with an integral approach to the architectural, structural and mechanical systems, all synthesized within the geometrical rules. The cells of the building can be rearranged according to different site and program needs when reused in the future. The basic building cell is a 40sqm hexagon module, with an inverted umbrella structure standing on a single pipe column that double functions as the rain flue. Rainwater is collected for landscape irrigation. 3 types of cells are created, transparent, enclosed, and exterior ones, to accommodate different functional needs. All building components’ connection details are designed to be reversible, no welding or glue allowed.