"Never forget" became a national rallying cry after the attacks of Sept. 11, 2001.
Yet America's schools — where collective memory is shaped — are now full of students who never knew because they weren't alive then. Many teachers now struggle with whether and how to teach the attacks and their aftermath.
According to one survey, only about 20 states include anything in depth about the events of that fateful day in their high school social studies curriculum.
And when they are taught, critics say, it's often through a narrow lens.