On Feb. 14, 2018, a gunman murdered 17 people and wounded 17 others at Marjory Stoneman Douglas High School in Parkland, Florida. David Hogg, who was 17 at the time, survived and helped to create March for Our Lives, which promotes gun control legislation. Now a senior at Harvard, Hogg tells correspondent Rita Braver what he has been through the past five years, including death threats against himself and his family; what his group has accomplished; and what it means for young people having to become leaders as advocates for change.