I was 9 years old, in my second week of middle school, 25 miles from lower Manhattan when the world changed.

I remember being gathered on the carpeted floor of our homeroom when the principal’s aide delivered a note to our teacher. She read it to us with a perplexed look on her face, unaware that the plane she described as having been involved in a tragic accident had passed just a few miles from our school along the Hudson River only minutes earlier.

We’d soon learn this was no accident.

9/11 MUSEUM CEO REFLECTS ON LASTING IMPACT OF TERROR ATTACKS AS AMERICA MARKS 250TH BIRTHDAY

We were too young to understand the magnitude of what was happening, but old enough to know something was terribly wrong. For millennials, September 11, 2001 was the moment that divided our lives into “before” and “after.”

There is an innocence in the photographs and memories from before 9/11 that is hard to explain. The people and the streets look the same, but everything feels different because we know what they did not yet know was about to happen.

MASSIVE PIECE OF 9/11 HISTORY MAKES STUNNING 10,500-MILE JOURNEY ACROSS AMERICA

And in the weeks and months after that tragic day, everything changed. We remember the anthrax scares, the new airport security measures, and the fear that another attack could happen at any moment.

But we should remember something else, too: the extraordinary sense of unity that followed. For a kid growing up just outside New York City, the shared sense of patriotism and American flags seemingly everywhere is impossible to forget. In those days, before social media became a force for division and politics consumed so much of our national conversation, Americans seemed determined to stand together.

I LOST MY WIFE ON 9/11. PLEASE LET THIS 25TH ANNIVERSARY BELONG TO THE FAMILIES

I still vividly remember watching President George W. Bush’s first pitch before Game 3 of the World Series at Yankee Stadium a little over a month after the attacks. In a city still grieving, our leader in a bulletproof vest humming a strike right down the middle felt like a small but powerful sign that America was still standing, too.

But 25 years later, September 11 is becoming history rather than memory. For a younger generation, it is increasingly just a chapter in a textbook—or worse, fodder for internet conspiracy theories, memes and jokes.

TUNNEL TO TOWERS ANNOUNCES ‘STEEL ACROSS AMERICA’ TOUR TO MARK 25 YEARS SINCE 9/11 ATTACKS

We cannot let that happen.

As the last generation to have firsthand memories of September 11, it is incumbent on us to remind younger Americans of the extraordinary things ordinary Americans did when confronted with the unimaginable that fateful day.

If you’ve visited any of the three September 11 memorial sites — the National September 11 Memorial & Museum in New York, the Pentagon Memorial or the Flight 93 National Memorial in Shanksville, Pa. — you understand why remembrance matters.

Nearly 3,000 people died that day. They were parents, children, spouses, friends, firefighters, police officers, office workers and travelers.

Innocent people who had simply shown up for a normal day at work were suddenly forced to make horrific decisions that no person should ever have to make.

And for countless others, the consequences of that day never ended. First responders and others exposed to the toxic air at Ground Zero have spent the decades since battling cancers, respiratory illnesses and other health problems tied to September 11.

There is no shortage of stories from that day. Some are harrowing stories of survival. Others are stories of heroism and extraordinary courage.

There was Betty Ong, the American Airlines flight attendant who helped authorities understand that Flight 11 had been hijacked. There were the passengers of Flight 93, who fought back against their hijackers and prevented the plane from reaching Washington.

CLICK HERE FOR MORE FOX NEWS OPINION

There were Stanley Praimnath and Brian Clark, strangers who miraculously helped one another escape the impact zone of South Tower after it was struck. And there was Rick Rescorla, the Morgan Stanley security chief who evacuated thousands from the South Tower before returning inside to help others and never making it out himself.

We were the children who watched the towers fall and the teenagers who grew up in the shadow of the two wars that followed. We are now the parents, teachers, and citizens responsible for explaining to the next generation why September 11 mattered.

We need to tell them about the first responders who ran toward danger while everyone else ran away, the passengers who refused to surrender, and the strangers who helped strangers.

And we need to tell them why we must never let it happen again.