It's hard enough for adults to understand. But for those who weren't born yet, the day the Twin Towers came crashing down is even more difficult.
"They ask what happened and why would somebody do something of that nature, why would somebody be so mean and cruel to really destroy buildings and destroy people," said Fullerton resident Kim Beck.
For some of them, a memorial of handmade crosses is how they will learn about the historic American tragedy -- about patriotism, recovery and the American spirit.
"The first year I had over 200 flags left to my memorial, and then since then I've been thinking 'I need to do more, I need to do more, I need to do more. How do I do more?'" said Santa Ana Firefighter Scott Townley, who designed the memorial.
Townley started the tradition a decade ago. Now every name of those who perished that day is handcrafted in its special place: Every firefighter, every police officer, every civilian.
"I made a point of making every single cross, individually, one by one, painted them one by one, put each name on one by one, and that way I got to know every single person," said Townley.
Every year, people swarm in to pay their respects, leave flags and memorabilia.
"After 9/11 happened everybody was all patriotic and as time goes people forget the date and I just think it's amazing he put so much time and effort into it to remind us all," said neighbor Jennifer Lamar.
"I was like a little kid last night. I could not wait to put up the memorial. I wanted this, I've been waiting for it for 11 months," said Townley. "It was 11 months of hard work and I'm really glad I was able to do it."
The memorial will be going up until Sunday, when there will be books and information about 9/11 set up and commemorative bracelets will be given out.
If you'd like to stop by and pay tribute or set up a flag, the home is located at Woods Avenue and Fern Drive in Fullerton.