In this Spark video produced by KQED, tour John Held Jr's expansive collection of mail art, one of the most extensive in the world. For more than 25 years, Held has been collecting art sent to him through the mail, and the collection amounts to well over 30,000 pieces. He has since donated part of his collection to various art institutions, including New York's Museum of Modern Art, the J. Paul Getty Foundation and the Smithsonian's Archives of American Art, but Held still keeps much of his collection of correspondences.