CHICAGO-On a sunny Sunday afternoon, Molly Sheridan is hard at work in front of a Starbucks coffee shop in Chicago.

The 13-year-old is playing her ukulele and, along with her five-year-old sister Edie, singing about Girl Scout cookies - boxes of which she has arranged for sale on a table. "Singing with my ukulele, I think that brings in people," Molly says.

It is Girl Scout cookie season, a uniquely American tradition marking its 100th year. This time of year is eagerly anticipated by the millions of Americans who crave the sweets that can only be purchased a few weeks each year, and can't be found in stores. Some fans take home a pile of boxes and squirrel them away in the freezer to enjoy all year until cookie season rolls around again.

The cookies aren't made by the scouts, but rather are marketed and sold by members of the service organization between January and April - the local weather determines the timing - to raise money for their activities.

The tradition has been interrupted only during World War II, when a shortage of ingredients led the scouts to sell calendars.

This weekend, the Girl Scouts will celebrate self-proclaimed National Girl Scout Cookie Weekend, and scouts will be out in force selling the 12 varieties of cookies.

"Most people know their favorites," Molly said, proven right just moments later by Anthony Stratton, 27, who stopped by to pick up several boxes.

His favorite: the Samoas - a rich treat covered with caramel and toasted coconut.

"It just has a yearly draw to it," Stratton said, "Knowing that the cookies come around once a year, it's pretty special then."

Jean Niederman, 56, stopped by to purchase Thin Mints, chocolate and mint cookies that are among the most popular.

"My older brother loves Thin Mints, and these are his 60th birthday present," she said.

Niederman can remember back to her own childhood, selling the same cookies door to door in the late 1960s. Back then, they were $1 per box. Now they're $5, she pointed out with a hint of irritation.

"Girl Scout cookies have always been popular," she said.

There are other differences today: a new box design, and a Scouts-specific digital selling platform that allows girls to sell to people anywhere in the United States, its territories and even on military bases in other countries.

Sales on sites such as Amazon and eBay are prohibited. Scouts are supposed to maintain one-on-one interactions with their customers, so they must use the Scouts website and personally invite customers to buy online.

Still, boxes of cookies can be found on e-commerce sites for a markup - at times twice their original price.

The Girl Scouts organization says it sells some $800 million worth of cookies a year. The girls keep the proceeds of what they sell.

Last year, Molly sold 1,500 boxes, raising hundreds of dollars to go on a nature trip with other scouts.

While that's a lot of sugar, butter and flour, she's not the most prolific saleswoman.

Katie Francis of Oklahoma City sold 22,200 boxes in 2015 alone.

The 14-year-old has sold more than 85,000 boxes overall - one of the best sales records at the Girl Scouts.

"It really takes a lot of time, and I've learned that attitude is everything," Katie said, detailing real-life lessons in entrepreneurship, service and resourcefulness.

"I've learned not to set limits in any goal," she said. "With time and determination, anything is possible."

That's just the kind of message that Sylvia Acevedo wants to impart.

The CEO of the Girl Scouts, a former scout herself, said selling cookies changed her life - instilling a young girl from modest means with a sense of self-determination.

"It's not just about selling cookies, it's about setting goals," Acevedo said. "It's really their first chance to start their own business."

Acevedo points out that nearly half the women entrepreneurs she encounters were Girl Scouts in their youth.

"It's building the girls' confidence," she said.

Molly said selling cookies has indeed helped her confidence and taught her to plan ahead. She plans to save her cookie sale earnings for two years to pay for a special trip.

"I'm excited to go to India in 2018," she said with a smile.