There once was a building on the outskirts of a big city. The building had been troubled with water problems for a long time. The apartment on the eleventh floor was the most troubled. During the nights, the comrades of the building gathered to solve the water problem. Some nights, they called in a plumber. Then everyone worked according to the instructions of the plumber. Even on this night, someone had come up with the final water solution, like every other night. The comrades of the building and the plumber worked for several hours. When the system of water pipes was complete, they turned on the water motor on the eleventh floor. Nonetheless, the two water tanks on the roof haven’t been filled this night either.