A case filled with empty Zyklon B cans is a haunting reminder of the poisonous gas used by the Nazis for killing prisoners on a massive scale. "When the gas chambers were full, an SS man put on the gas mask, went to the roof, opened the little window there and threw such a can into the gas chamber," Professor Wiesel explains. "Unspeakable pain and horror—that's how they were killed. Mothers and children hugging. ... The death factory became industrialized and industry worked well."