We can all remember exactly where we were when the two towers came crashing down 15 years ago.

Sept. 11, 2001, is a day seared into the collective memory of our nation — and also the individual memories of everyone who watched the horror unfold on live TV.

But for the first time, a group of high-schoolers are learning about 9/11 as a piece of history that took place before they were born. This year’s ninth-graders weren’t yet born when two hijacked airplanes struck the towers in New York City.

To them, it’s history the way Pearl Harbor is history to many of us.

The challenge for us as Americans is making sure those young students — and the generations that follow — understand the impact that event had on this great nation. Their lives have been shaped by that act of terrorism, whether they realize it or not.

They have never known short lines at the airport, or joining a loved one in the terminal without a ticket. They have never known a world where terrorism isn’t always in the back of everyone’s mind anytime a tragic event occurs. They haven’t known a government that isn’t actively fighting terrorism around the world on a daily basis — or conducting mass surveillance. Sadly, they may never know an America as united as it was in the weeks that followed the attacks.

They haven’t known the relative sense of safety Americans felt at home. Not since Pearl Harbor had America been attacked like that, and most of us wrongly assumed it could never happen again.

This year’s high school freshmen mark a significant crossroads in how we remember the events of that day. Schools no doubt spent time recently learning about the event. Students saw photographs, read news stories and maybe even watched footage of that horrible day. They will certainly know about the events that unfolded.

But they will never feel them the way those of us living at the time did. They will never know the stomach-churning fear that we all felt. They will never understand the enormity of the loss – both in life and sense of security.

But it’s up to those of us who do understand to share with them. Not to scare them, but to help them understand how a single event so drastically altered America.

It’s also up to us to convey the sense of pride and patriotism we felt as thousands of Americans traveled to New York City to dig through rubble, donate blood or just lend a helping hand.

American flags lined the streets of just about every city and small town following the attacks. For a brief period, Republican or Democrat meant nothing. The only label that mattered was USA.

As we remember the tragic events of that day — and the thousands of lives lost — let’s also remember the unity that followed.