The man who spends most of his life in motion sits still, pausing over the question.
Carefully considering his words, he begins reconstructing his memories of a day he'd rather forget. It was somewhere just past Fenway Park, amid the race-day chaos of Kenmore Square, that Louie Cap heard that sound.
"I thought it was fireworks," Cap said, a distant look on his face.
It could have been anything at that point, about 25 miles into his 38th consecutive Boston Marathon, when the senses start to slip and adrenaline and force of will are all that's left to carry you the rest of the way home.
Moments later, police cars zoomed by, cutting past the bedraggled crowd of the almost-finished. They were coming fast, too fast for anything good to have happened, toward the finish line that for the first time in nearly four decades, Cap would never reach.
Did he see first-hand what television cameras would soon capture: the madness and blood and fear after a pair of bombings rocked the city's proud annual celebration of athletic achievement?
Again, Cap hesitates.
"That was horrible. I saw all the -- oh, it was horrible."
Almost a year later, the nuclear physicist is shouting instructions, lobbing one tennis ball after another to a pair of awaiting students. Dressed in a bright blue shirt, white shorts and matching hat, eyes shaded by stylish sunglasses, the 69-year-old Cap is winding down for the morning. He bids his students farewell and eases his way over to a chair in the shade.
This is how Cap spends his days, as a tennis instructor at the Van der Meer Racquet Club, mentor to vacationers and protègès alike. On Monday, he'll be back in Boston to run his 39th consecutive marathon, and he intends to finish this time.
The Boston Marathon, more than any other race, has a special place in his heart. He says he'll run this one with few thoughts of last year's tragic violence. It's easy to take him at his word. If you know anything about Louie Cap, it's that he won't ever quit.
Cap was born in Czechoslovakia in 1945, and grew up during the height of the Cold War. Education was always important, but the best opportunities were on distant shores. Cap came to the United States around Christmas of 1966. Then 21, Cap said the idea of staying was already on his mind.
"The thought was there, for sure, because of the situation those days," Cap said. "I never really intended to come back."
In 1973, he earned a Ph.D. in nuclear physics from Baylor University, but at that point, Cap was forced to reassess his career options. He was unsure he'd ever get the security clearance needed to work for the U.S. government, lacking full citizenship upon graduation from Baylor. That came in 1976, but not before Cap jumped at the chance to work with future tennis coach Dennis Van der Meer, whom he met on a trip to Berkeley, Calif.
Ever since then, Cap has spent time traveling the world, meeting famous players like Billie Jean King -- a former student of Van der Meer -- and eventually teaching the game to others.
Of course, he insists he never left his physics background behind. He's been known to launch into discourse about the spin on a tennis ball, the difficulties of transferring enough power to counter topspin, and the scientific reasoning behind older players preferring to perfect a slice technique -- it uses less energy.
Some of his students know about his esteemed academic background, information gleaned through Internet searches or admission on Cap's part, but he is too humble to encourage others to use the formal prefix of "doctor."
"This is America," Cap laughed. "The fact that I have a Ph.D. doesn't make me any better than anybody else. If you're teaching because of the money, you might as well sell cheese."
Cap's love affair with running was born of embarrassment: Van der Meer blew past him on their first jog. The admittedly competitive Cap then trained in secret until such a time as he could beat his friend in their next race.
The Boston Marathon was always a dream of his. Thirty-nine years and one incomplete race later, Cap is eager to get back onto the long path through Boston, urged on by what is sure to be an overflowing, emotional crowd.
This is what makes the marathon so special, even after so many successful years. Cap takes pride in his fitness, in his ability to push himself to and over mental and physical barriers. The course is famously challenging -- including the infamous Heartbreak Hill -- but Cap said what makes the Boston Marathon special, and will continue to make it special in the shadow of tragedy, are the people who line the route and cheer on everyone who goes by.
"It's a day made for you," Cap said. "The city lives for all the runners one day of the year."
He won't be running scared, either.
"For these two guys, how many thousands have to suffer?" Cap asked. "My thing is run it, enjoy it, every second of it. You never think twice. It's like everything. I just try to live life to the fullest every day."
He said people keep asking him when he will retire. He looks at least 10 years younger than he is, with a trim physique, a well-established tan and a consistent smile. Cap is incredulous over questions about his age and his future. He plans to run a few more marathons yet, and certainly intends to keep playing tennis.
"How can you retire in a job like this?" Cap asks, looking up at the bright morning sunshine. "It's paradise. You love what you're doing, you have to be busy, do something active. How much better can you do than this?"
THE 118th BOSTON MARATHON
WATCH LIVE ONLINE
Monday's Boston Marathon will be streamed live online for free at http://watchlive.baa.org/ starting at 9:30 a.m.
Mobility Impaired 8:50 a.m.
Wheelchair Division 9:17 a.m.
Handcycles 9:22 a.m.
Elite Women 9:32 a.m.
Elite Men & Wave One 10 a.m.
Wave Two 10:25 a.m.
Wave Three 11 a.m.
Wave Four 11:25 a.m.
Ludovit S. Cap, Hilton Head Island
Ed Fishel, Hilton Head Island
Jeffrey Ford, Bluffton
Glenn R. Lankowski, Bluffton
Robert B. Maxwell, Beaufort
John Tolerton, Bluffton
Benjamin Vaught, Bluffton
Elizabeth A. Beller, Hilton Head Island
Kim Busby, Hilton Head Island
Beth-Anne Canero, Beaufort
Aileen C. Cangiano-Heath, Beaufort
Denice Davis, Beaufort
Pam Drafts, Beaufort
Kirsten A. Fishel, Hilton Head Island
Donna Garske, Hilton Head Island
Amie S. Lankowski, Bluffton
Lindsay M. Martin, Bluffton
Kristen E. Rider, Beaufort
Kendra Twitty, Yemassee