Ask almost any American adult where they were on September 11, 2001, and usually they will share a vivid memory of that epoch-defining day.
But for today’s schoolchildren, 9/11 is a historical event that happened long before they were born and the coming 20th anniversary of the deadliest terrorist attacks in US history has little personal significance.
Twelve-year-old Lukas Welter from California, for instance, said 9/11 was a time of “sadness and hope”, but he acknowledged he did not fully understand what had happened, even though his father liked to share his personal recollection of events.
“My dad was on the plane behind Flight 93 and Flight 93 took off one minute before my dad’s flight,” Lukas told The National. “Every year on September 11, he posts his boarding pass on social media of that flight that he took.”
Flight 93 was one of the four hijacked planes used in the attacks. It crashed in rural Pennsylvania before reaching its supposed target of the Capitol building in Washington.
Despite the family story, Lukas said, “I didn’t know how to feel because I wasn’t alive back then. I’m kind of unknowing about what actually happened."
In the heart of lower Manhattan on a large plaza where the Twin Towers once stood, the National September 11 Memorial & Museum aims to give visitors of all ages a broad understanding of the day.
It is bustling with families. Parents talk their gangly tweens and teenagers through the attacks and pause to read the names of some of the nearly 3,000 people who died.
Joseph Reyes, a middle-school pupil from Colorado, said what stood out to him when he visited the site was “how many lives were lost".
At school, he wants to “learn about why they did it and how [the US] fixed it and how they got back on their feet”.
Though he never formally studied 9/11 in his native Peru, 18-year-old Santiago Alvarez said touring the memorial brought on “a lot of feeling for me”.
Continuing to remember the attacks was “important for America and for the world”, he said.
But US schools are not mandated to teach the history of 9/11 and there is no single national curriculum, only a set of academic standards that are voluntarily adopted by states.
That means it is often up to educators to decide whether and how they want to teach 9/11, said Hadley Sucher, an elementary schoolteacher in Brooklyn. She shows her youngest pupils simple story books on the anniversary.
“I tell them, today is a sad day in the history of our country,” she said.
“Then they ask, why would they do that? Why would they crash the planes they were in?”
Ms Sucher believes teaching the history of 9/11 is vital, especially where she works – a neighbourhood with many Muslim families, as well as families of firefighters and first responders.
“It’s still relevant to their lives,” she said.
Younger children typically learn a basic chronology of the events of 9/11, Ms Sucher said, then progress to the repercussions of the event, such as discrimination and the wars in Iraq and Afghanistan, later in high school.
The Memorial & Museum also works with schools across the US to offer webinars and powerful first-person accounts that help children process the emotions of 9/11. But, while informational, its exhibits may give a skewed picture of the attacks, one activist said.
“If you look through their educational materials, you will see that in nearly every one, they are chiefly focused in defining Al Qaeda in terms of religion and 'Islamism', without the necessary political and historical context,” said Todd Fine, president of the Washington Street Advocacy Group, which promotes historic preservation and memory in Lower Manhattan.
“The result is a danger that visitors and students will blame all Muslims.”
Teenagers that know 9/11 is a complex subject are frustrated not to learn more at school.
Fifteen-year-old Vaughn Schweiger said teachers at his Texas high school “don’t do it justice, in my opinion”.
“They really just told us that two planes crashed into the Twin Towers and that’s about it," he said.
"I think they should let us know everything that happened and talk about Al Qaeda more, the terrorists, and how it affected us as a country in general, not just the fact that two planes crashed into two tall towers.”
Vaughn’s sister, Ava, said “people should care about it and try to learn more about it”.
Some educators feel 9/11 is becoming less and less relevant to their pupils, even though the attacks still frame much of US foreign policy.
You need look no further than the chaos engulfing Afghanistan to see the continued relevance of the attacks.
Cindy Mauro, a French teacher in the Bronx who was only two days into her job when the attacks happened, said she believed the importance of the attacks has faded over the past decade, even for people who were in school at the time.
“You go from these kids having seen it, having lost family members, to it being like the Civil War [1861-1865] for them,” she said.
Carlos Williams, 49, a former history and social studies teacher in Maryland, said "at the time it was a paramount issue, but history evolves".
Children born after 9/11 “have lived through school shootings. That is more relevant to them”.
On the 20th anniversary of the attacks, many feel 9/11 education remains a priority.
“It is like a reminder of all the brave people and what they do for our country,” said 15-year-old Rebecca Kenville from Iowa.
She said 9/11 explained things about the way we experience life today.
“If they stop teaching, I think that is disrespectful and I think kids need to know why we take all the precautions we do when we fly," she said.