In 2008, an earthquake caused the collapse of an elementary school in Sichuan, China.  Ai Weiwei conducted an investigation into the government's responsibility in the construction of the school, including choice of building material and civil engineering laws, and their cover-up of the deaths that resulted from their oversight.  For months, Ai spent his time interrogating government officials and researching the names of children who passed away in the incident, attempting to determine how many children were lost that day.  Ai created Remembering, made out of children's backpacks, as a tribute to the 9000 children killed in the earthquake, with each backpack representing a child who was killed.  The red, yellow, and green backpacks spell out a quote from a mother whose child was lost that day: "She lived happily in this world for seven years."  This piece of art, showcased in his retrospective So Sorry in Germany, is attributed as the piece that led to his arrest in 2011.