Why do we mark the anniversaries of tragedies? Two experts on this topic join us. Stewart Hoover directs CU Boulder’s Center for Media, Religion and Culture. He wrote about memorializing 9/11 in a book called "Religion in the Media Age." Also with us: Barbie Zelizer. She’s a professor of communication at the University of Pennsylvania. She studies times of crisis and how we remember those times. Her newest book is “About to Die: How News Images Move the Public.” Hoover and Zelizer speak to Ryan Warner.