The Pentagon is home to the world's most powerful military, which runs secretive and dangerous defense operations around the world. And yet the Pentagon opens its doors to the public (100,000 visitors annually) - for seven tours every weekday.
"About 23,000 people work here on an average day. This building does function a lot like a mid-sized city," explained Seaman Marie Reinert, who has been giving tours at the Pentagon for 17 months.
Earning the stripes to guide visitors isn't easy. Candidates are hand-picked from the military's prestigious ceremonial guard. Then they must memorize a 35-page script in just 15 days - only 55 percent make the cut.
"I think the best day was when I passed training," Reinert said. "I didn't think I was going to pass."
But pass she did. And she knows the facts. Reinert can tell you that there are 691 drinking fountains, 19 escalators, 131 stairways, 284 restrooms and 16,250 light fixtures in the Pentagon.
Reinert also has a flawless ability to keep the group engaged by combining lighthearted jokes and mind-boggling facts with descriptions of historically significant events.
"If you look straight up above you, you will actually see the Pentagon's five floors above the ground," she told a tour group, gasps audible, as the group looked up and saw the multiple escalators that delivered them to this location.
"Now, if you look straight down you will see... your feet," Reinert quipped.
"I got like half of you," she said with a laugh.
"Now we are going to be making our way to the point of impact of the terrorist attack on September 11th 2001," said Reinert, transitioning into a historical event that deeply affected the country's psyche and national defense.
The indoor 9/11 memorial has the names of the 184 people that died in the Pentagon that day and there is a book where visitors can write their condolences to the families of those individuals.
Not every part of the tour has such emotional depth: just around the corner, a fake rat stared into the visitors' eyes. The rodent is part of a display of what the Pentagon offices looked like in the 1940s when the building suffered from a rat infestation problem. Reinert promised the group that this problem no longer exists.
During the tour, Reinert fielded questions and kept her eyes on the group by walking backward from one hallway to the next.
And, yes, this backward stride is part of the training.
"When I first started, I backed into a person. He was actually holding a cup of coffee at the time," Reinert said with a jolt of mirth in her voice. But that run-in didn't slow her down.
In a building dominated by men, she is one of just four female guides. What does she think of that? "It actually makes me work harder," she said, "I love a challenge."
"I have changed completely being here as a person," Reinert said. "I have become a lot more confident with myself because I am talking in front of thousands of people every year. I have been able to be myself and be confident with the person that I am. It has made me an exceptional leader."