Many news outlets report that last weekend's shooting in Las Vegas is one of the deadliest in modern U.S. history. We take a moment to consider our country's history of mass casualties, and what constitutes as a "mass shooting" by definition. Plus, how active shooter training in school is changing for kids as gun violence is on the rise. Guests: Philip Duncan , linguistics professor, KU Grant Duwe , author, A History of Mass Murder in the United States Erin Hambrick ​, psychology professor,