CHICAGO - When Liz Hletko goes toy shopping for her three children, finding just the right item for her son is a snap, while the same task for her two daughters is anything but child's play.

"Everything that is marketed to my son calls for using his brain," said the Evanston, Ill., mother, whose kids are 7, 8, and 10. "But for my daughters? It's all about beauty or taking care of something."

It was a common lament as parents of girls scoured the pink aisles this holiday season in search of something that wasn't frilly or sparkly. But the issue is generating new heat, thanks to a video that went viral since it was posted on YouTube last month. It touts a line of toys called GoldieBlox that is designed to spark young girls' interest in building and inventing. Its motto? "More than just a princess."

Parents have embraced the GoldieBlox message of inspiring girls to enter the science, technology, engineering, and math fields - STEM, in academic parlance - turning its $30 Spinning Machine toy into a hot seller on Amazon.

GoldieBlox joins other new construction-related products aimed at girls - such as Lego Friends and Roominate, a dollhouse with circuit boards - even as researchers try to figure out why girls aren't choosing math and science careers.

"In other countries, you don't see this big disparity," said Cindy Menches, an assistant professor of engineering at the Illinois Institute of Technology. "What else could it be but cultural differences?"

Researchers say boys and girls start out with an equal appetite for math and science, but by middle school, girls' interest ebbs and the gender gap widens. Space camps and science fairs suddenly morph into "boy" activities, and by high school, girls are noticeably absent from subjects such as calculus and physics.

Myriad factors are likely at play, but GoldieBlox has put the spotlight on toy stereotypes. Even preschoolers quickly grasp which aisle is geared to them, child development experts say, with girls reaching for the pastel packaging, while boys seek out dark hues on items with wheels, pulleys, and robotics.

Debbie Sterling, GoldieBlox's founder, said she has nothing against princesses; she just thinks they should build their own castles.

The 29-year-old graduate of Stanford University started the Oakland-based company in 2011, having spent a year researching gender differences in toys and interviewing everyone from neuroscientists to kids. Girls liked building things just as much as boys, she found, but they also loved stories and characters and wanted to know the "why." So Sterling replaced the usual manual with a narrative relating to girls' lives.

Hletko, a psychologist, was just relieved to see anything that points to more tool belts, less tulle.

"I like that focus . . . that girls can make and do things," she said.