(NewsNation) —  After the devastation of 9/11, one teacher, Elaine Mroczka, decided to have her group of third-graders write letters of comfort to a New York classroom.

Now, 20 years later, she’s kept in touch with the New York teacher, Julie O’Connor, whose students received the letters.

O’Connor said she still remembers going to work on 9/11, which started out as a “beautiful” day with “pitch-perfect weather.”

Then, the terror attacks happened.

“I had a teacher come in saying that a plane went into the building downtown, and then another plane went in, and we realized we were under attack,” she said.

Parents came in, asking to pick up their children, and O’Connor’s husband wasn’t able to reach her right away to let her know he was OK.

“It was very frightening,” O’Connor said. “We were just shocked, and there was a lot of fear. So much was about to change for New York City kids and our surrounding areas.”

The days after were a very “fearful time,” O’Connor recalled. So Mroczka’s letters came at just the right moment.

“We felt just so comforted in that,” O’Connor said.

Even from miles away, Mroczka felt compelled to help the New York classroom, because, “We all felt that pain” from 9/11.

“We wanted to let them know, people are Americans, people care,” Mroczka said.

The students kept communicating as pen pals for the next year, and the two teachers have remained friends in the decades since. They’ve even written a book together about their experience, called “United States of Friendship: Pen Pals of 9/11.”

From all of this, Mrozcka says she’s learned the importance of helping others.

“It’s just a simple act of kindness, a smile, opening a door for someone, calling someone who’s alone, and needs some help or some encouragement, just simple acts of kindness,” she said.”If everybody did that, every single day of their lives, the world would be a better place.”