Hi, I'm Dan Barry, a writer for The New York Times. Here's my personal essay. What does it mean to never forget 9-11?

In his mind, Michael Regan should have been down there. He should have had the guts.

A long time New York City employee who became First Deputy Fire Commissioner after the terrorist attacks of September 11th, 2001,

Mr. Regan coordinated scores of funerals and memorial services and helped hundreds of shattered families. Still, he could not shake the guilt. He should have been there down at the World Trade Center.

After a couple of months, Mr. Regan finally shared his remorse with a stunned fire department colleague who told him that he had been there.

He'd helped transport the bodies of the First Deputy Fire Commissioner Bill Finn and the Chief of Department Peter Gantzie to the Morgan First Avenue.

Don't you remember?

Looking back, Mr. Regan said his mental block must have been a way to cope with the instant loss of thousands, including many close friends.

It was a safety mechanism, he said. I saw horrible things that day, and I didn't want to think about those things.

20 years later, the command to never forget retains its power, jolting a sense of the past whenever we see it on a hat or flag, or the back of a passing car on the belt parkway.

For all its slogan-like simplicity, these twinned words seem freighted with the complexities of guilt, obligation, and even presumption as if we could ever forget.

But now that an entire generation has been born since the day, versions of the question posed to Mr. Regan might be asked of all of us who lived it in some way.

The complaints hijacked by Al Qaeda piercing the north and south towers of the World Trade Center, a third slamming into the Pentagon and Arlington, Virginia, a fourth crashing in an open field outside Shanksville, Pennsylvania, all in less than 90 minutes.

What exactly do you remember? What stories do you tell when a casual conversation morphs into a therapy session? What stories do you keep to yourself?

And what instantly transports you back to that deceptively sunny Tuesday morning?

For Nikki Stern, a writer, it might be the waft of cigar smoke. Her husband, Jim Petority, a vice president at Martian McClenan who worked on the 96th floor of the north tower, enjoyed the occasional cigar.

Or it might be the side of a bicycle, just a bicycle. Jim used to cycle.

I compartmentalize, Mr. and said, but there's a permanent leak in the compartment. For James Lwango, a former deputy chief of the New York Police Department, its driving past the now closed Fresh Kills landfill on Staten Island.

He spent nearly a year on that mound supervising a pop-up base camp where 1.8 million tons of trade-centered debris were sifted for human remains and personal effects.

The problem is Mr. Lwango lives on Staten Island.

You've got to put it where it needs to be, he said of the memories, and not open the door more than you have to.

Never forget.

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