Howard Mansfield: 'A monument works, when they're call and response'

Howard Mansfield is author of "Invisible Monuments - Tribute, Memory, and the Summoning of the Past" joins Megan Lynch. Why do some monuments stick with us? Why don't others? How have they evolved over time? 'We've lost that commemorative language' says Mansfield of statues of men on horses, since people don't rely on horses anymore.