Memorials are symbols that a nation has not forgotten— and will not forget—an event. They can be permanent symbols, such as statues, tombs, pyramids, walls, or arches; or intangible ones, such as lists of names or a collection of photos on the Internet. Spontaneous memorial sites are familiar around the world, from the crosses that are often erected at scenes of fatal accidents to the massive collection of flower bouquets left at the American embassy in London following the terrorist attacks on September 11, 2001. The memorial in Oklahoma City started out as an impromptu site, with stuffed toys, flowers, and personal messages for the victims—or to no one in particular—left at the chainlink fence that surrounded the destroyed building. The spontaneous outpouring of grief allows people to show that those who lost their lives were valued and loved and...