Sandy lives on the top floor of a 10-story building in a big city. There is a 30-story apartment building under construction next door to her building. One morning while eating her corn flakes, a brand new toilet comes crashing through the roof of Sandy's apartment and injures her. She would like to sue the construction firm, but has no way of finding out or proving what persons were involved or exactly what happened. The doctrine that would most help Sandy is ??