Boston and gunpowder were already linked in my memory.

I lived in Boston for a while when I was in high school—in Arlington, to be exact, which I used to describe as “the town where old people go to die,” because there were three funeral parlors within two blocks of our apartment. From Arlington it was a short bus ride to Cambridge and Harvard Square, and then a few subway stops to downtown Boston. I spent more time in Harvard Square than in Boston, because heckling the Cambridge street preachers was so much fun, but I also recall hanging out around Boylston Street on more than one occasion when I should have been in school.

Patriot’s Day, the third Monday in April, also stands out in my memory, in part because it’s a state holiday in Massachusetts, and thus gave me a legitimate reason to be out of school. In 1775, Arlington (called Menotomy back then) saw heavy fighting during the battle of Lexington and Concord. Commemorating that conflict is still a Big Deal in Arlington, which celebrates Patriot’s Day with parades and speeches, and fireworks in the evening.

My father was a historical re-enactor, so our house saw a lot of musket-cleaning and cartridge-filling the weekend before Patriot’s Day. Dad used to let me him help roll cartridges. He would cut sheets of newspaper into almost-square shapes, with an angled edge, and I’d roll them up around a wooden dowel, twist one end, and crush it flat against the tabletop. Then Dad would measure out the black powder and pour it into the paper tube, and I’d twist the other end tight and fold it down.

Unexploded gunpowder has very little smell, unlike the eggy, slightly metallic scent of the smoke that wafted over Spy Pond in the evening from the Patriot’s Day fireworks show. Measuring and pouring it is much like any other handicraft project, provided there’s no open flame in the room. I found the process strangely soothing: Just the two of us sitting peaceably in the living room, father and daughter, stockpiling ordnance.

Dad would put the finished cartridges in a leather shoulder pouch, and on Patriot’s Day when he was re-enacting the battle at the Jason Russell House a couple of funeral homes away, he’d grab the paper tubes one at a time, tear them open with his teeth, pour a dash of powder into the priming pan of his flintlock musket, empty the rest down the barrel, shove the paper in after for wadding, tamp it all down with the ramrod, aim at one of the ersatz Redcoats swarming the field, and fire.

The only thing missing was the bullet. Dad occasionally bought lead shot for target practice and shooting competitions. But he and I never rolled any cartridges with musket ball for Patriot’s Day. The carnage of Lexington and Concord belonged to the distant past; we were just evoking its memory.

I can easily imagine Tamerlan and Dzhokhar Tsarnaev sitting around their Cambridge apartment the weekend before Patriot’s Day this year, relaxed and chatting as they extracted black powder from fireworks, then measured and packed it carefully into pressure cookers. I can almost hear the soft hiss of sifting powder, the quiet murmur of companionable voices. But the Tamerlans weren’t commemorating past violence, so they also loaded their handiwork with nails and ball bearings that would tear through the crowd of spectators at the Boston Marathon.

Two hundred and thirty-eight years have passed since the Revolutionary War started in Boston, but gunpowder is still gunpowder, men are still violent, and human bodies are no less vulnerable than they ever were.

A few days before the Boston bombings—perhaps as the Tsarnaevs were stopping at the hardware store for another package of nails—the university where I work received what the police called a “non-specific bomb threat.” They sent us an official email notice about it (some ten or twelve hours after the threat came in, when we’d all gone home for the day). The email offered no details, merely assuring students and staff that “there is no information at this time that indicates this is a credible threat.”

I didn’t find that lack of information especially comforting, maybe because I spend practically every minute of my working day acutely aware of how vulnerable I am to being shot or blown up. Consider: The basement office where I work lies next to a 200-seat lecture hall. Ten minutes before each hour, the atrium outside fills up with large herds of stressed-looking college students, waiting to troop into their classes. I can see them clearly because the front wall of our office is made of floor-to-ceiling glass. And if you enter the office through the glass door and walk a few steps past the receptionist’s desk, you’ll come to more floor-to-ceiling glass, which is the front wall of my personal office. The side and back walls of the room I occupy are made entirely of cement blocks.

If you’ve ever spent time on a shooting range, this configuration probably sounds familiar. If you’ve never been to a shooting range, here’s how they’re usually laid out: The range (that’s the “shooting room,” for you none-gun people) is isolated behind not one but two walls, and the upper halves of both walls are made of glass, so the staff can see every movement the shooters make. When you’re heading in to shoot, you open the first door and step into the antechamber between the range and the check-in area. This functions like an airlock. The first door has to close behind you before you can open the second one, and step onto the range proper: A long, narrow, bunker with concrete walls on three sides, and the glass behind you.

My office is laid out exactly the same way. I sit with concrete behind and on both sides of me, and I face the glass—that is, toward the shooter. True, the walls of my office are plastered and painted, unlike those at a shooting range, and there are some bookshelves and things. To date, there are no bullet holes. The only other difference is that, in place of a paper target hanging from a wire, there is the chair where I sit all day long, staring at my computer, swearing at an Excel spreadsheet.

I’m just about the most attractive target imaginable for a random gunman. And in the event of a bomb exploding near my little cement cell, they’d have to recover my remains with a Shop-Vac.

Personally, I find the constant awareness of my own mortality distasteful—doubly so when I’m on the clock. It’s distracting, for one thing; it prevents me from focusing on whatever I’m trying to do at the moment, whether that’s turning off Excel’s inane auto-correct feature that changes every number into a date, or reading pointless emails from the university police. It’s also demoralizing as hell. Maybe the two campus-wide lock-downs I’ve been subjected to in recent years have contributed to my sense of claustrophobia on the job. The tireless efforts of the Texas Legislature to make concealed weapons legal on campus certainly haven’t helped.

But any way you look at it, working in an office that could, with minimal effort, be reconfigured as a shooting gallery is a weird feeling. I dislike that feeling. I’ve always hated feeling defenseless, even before I took a job in a bunker. I’ve gone to great lengths throughout my adult life to reduce my vulnerability and increase my power—with karate, running, swimming, weights, pushups—so I can react faster and hit harder. I’ve trained with knives and sticks and guns, and studied anatomy and attacker psychology so I can choose the best line of defense against assault. I practice situational awareness, even when this means interrupting the people next door during their World Cup viewing party to ask what all the screaming is about.

But all these efforts to push away the reality of my vulnerability have only made me more aware of it. It’s true, I’m somewhat less vulnerable than I was when I started training in martial arts. Yet I’m still extremely vulnerable. We all are, no matter how fast we can run or how much we can bench press. Events like the Boston bombings bring that fact home in dramatic fashion, but I’m reminded of it every time I walk into my office.

The real struggle, I’ve discovered as I hunker down in my basement day after day, isn’t for safety. The real struggle is for balance—for strength on one hand and an acceptance of weakness on the other.

It’s easy, so easy, to hurt people. It’s easy to see vulnerability itself as a threat to be overcome. The hard truth is that genuine security—the kind that doesn’t lead to wide-eyed, ever-increasing paranoia—requires the courage to remain open to our own vulnerability. When that courage fails us, we inevitably start adding bullets to the cartridges we’re rolling.

Nine days after Tamerlan Tsarnaev was killed and his brother Dzhokhar arrested, I signed up for a 5K race in downtown Austin. I saw it as a small act of defiance, against terrorism and my pathetic VO2 max. This is how badly you’ve failed at frightening me, I thought as I registered. I’m putting myself in harm’s way for this race, and I can barely run an eight-minute mile.

A few minutes before the start, I trotted up a nearby hill to look out over the assembled crowd. Just as there had been in Boston, there were kids and parents and dogs, volunteers handing out water, police officers, and—a uniquely Austin touch—some poor drudge dressed in costume as the mascot of the restaurant sponsoring the event: A relentlessly cheerful, anthropomorphic sandwich. The street was crowded with runners and spectators, and almost everyone was carrying some kind of bag—a fanny pack, or a packet from the registration table holding their T-shirt and race bib. There could have been dozens of pressure cookers hidden in the baby strollers that dotted the pavement, or in the trashcans all around. The sandwich alone, I estimated, could have been hiding twenty pounds of black powder and shrapnel under his foam rubber bun.

I looked down on all those people and thought about Boylston Street, and to be honest, I felt a little queasy. But for once I felt no urge to fight the feeling. The destruction in Boston led me to understand something that is much harder to see from behind a desk. I would be ashamed to be invulnerable, I realized.

Because whatever compassion we possess springs from our vulnerability. And as much as I value my life, I value compassion more. How awful, I thought, if I were to look at all those people, and take satisfaction in the knowledge that they could not touch me in any vital way. You might as well be dead, really; or trapped in a concrete cell you can’t ever escape.

Tamerlan Tsarnaev—a boxer, a tough guy, a terrorist—is dead. His hapless little brother Dzhokhar will no doubt spend what’s left of his life in a concrete cell. They could not reconcile their vulnerability and their power. And their fate is a mournful reminder of how important that balance is for each of us.