Movie theaters have become a lot more pink since Barbie opened.

Fans of the iconic doll are stepping out to see the smart and funny, Greta Gerwig-directed romp in outfits that Barbie herself would certainly approve of. Their ensembles are bursting with life, every shade of the character's signature color, as well as sparkles, pretty patterns and ruffly skirts or tops. They're the kind of looks that make you feel like, as Mattel's theme song says, that you could do anything.

Andrew Liptak, author of the new book Cosplay: A History, tells Yahoo Entertainment that the most basic reason people dress as their favorite characters in situations like this is it's a form of self-expression, a way of telegraphing what you like and, therefore, a piece of who you are. It shows your devotion to a certain character or world.

"By dressing up," Liptak says, "you're able to bring that story to life a little bit."

Plus, it's just fun.

"It's a form of play, and it's a form of entertainment, and I think that… we sort of overlook the importance of play and to have fun and to relax and just enjoy ourselves," Liptak says. "And this is an outlet to do that."

Dressing up for such a movie also is a way of connecting with others — "It's fun to be in the camaraderie of like-minded people," Liptak says.

"Barbenheimer," the popularized term referring to the hotly-anticipated blockbusters Barbie and Oppenheimer opening on the same day, made us feel connected.

But unlike her theatrical counterpart, Barbie is extra fun, since Barbiecore is vibrant, shiny and, to quote the band Aqua, "fantastic."

In the case of Barbie, another thing driving the cosplay is nostalgia. Barbie dolls have been around since 1959, early enough that the grandmothers of today's children might have had one in their toy box too.

Human behavior expert Patrick Wanis explains that that certainly plays a part.

"Many children, specifically girls, grew up with Barbie and all her different versions," Wanis says, "so it has great significance when there is a movie. Here is an opportunity for an adult, who was greatly influenced as a child growing up by dressing up as Barbie or dressing up Barbies, and now they have an opportunity to relive a part of that childhood."

He adds that, while fun is a motivation, he believes it's even more about "relating and reconnecting through childhood," and grasping whatever Barbie represented to the person dressing up, whether that's beauty or power or creativity, even romance or freedom from responsibilities.

Whatever inspired them, creatively-dressed Barbie fans contributed to the whopping $162 million the movie took in at the box office opening weekend. Perhaps Gerwig will inspire a new Barbie doll, as it was the best domestic opening ever for a movie from a female filmmaker.

Unofficially, it was also one of the most fashionable film debuts. Here's a look at the wide range of looks spotted:

From left, Sophie Davidoff, Ayan Babayeva and Mia Ilyaich, all 9, wore pink to see Barbie on July 22 at AMC's The Grove 14 in Los Angeles. Babayeva was celebrating her birthday. (Photo: Jenna Schoenefeld for The Washington Post via Getty Images)

Over in Boston, a group of friends who are part of a summer filmmaking program at Emerson College, dressed up to Barbie for a July 20 show the AMC movie theater at Boston Common. (Photo: Vincent Alban/The Boston Globe via Getty Images)

The opening day of Barbie brought plenty of pink ladies to a New York City theater. (Photo: Stephanie Keith/Getty Images)

At the storied TCL Chinese Theater in Los Angeles, Cierra Fraser and Joslin Farley wore Barbie-inspired ensembles to see Barbie. (Photo: Jenna Schoenefeld for The Washington Post via Getty Images)

TChaka Cumberbatch, 36, poses on July 21 outside AMC Century City in Los Angeles. (Photo: Dania Maxwell / Los Angeles Times via Getty Images).

A group of friends dressed as characters in Barbie outside a movie theater at AMC Century City on July 21 in Los Angeles. (Photo: Dania Maxwell/Los Angeles Times via Getty Images)

Ken got some love, too, from Anthony Vaccaro, Wenwei Zheng, Oscar Beer and Michael Stratigakis. They caught Barbie on July 21 at AMC Century City in Los Angeles. (Photo: Dania Maxwell/Los Angeles Times via Getty Images).

Jasmine Vaughn Perrett said she loves Barbie because she loves a woman "who represents being beautiful and smart" before seeing the film in Los Angeles. (Photo: Dania Maxwell/Los Angeles Times via Getty Images).

Moms and daughters, such as Maria Plascencea and Daniella Galvan in Los Angeles, wore pink to the theater. (Photo: Dania Maxwell/Los Angeles Times via Getty Images)