Instructor Sarah Smith roams around giving one-on-one instructions.
Smith said that each hoop has its own rhythm; that the smaller ones fly around the body really fast.
"It helps to get a huge huge hoop when you're first starting out so you can feel where to push and pull," she tells a first-time student.
Hula hooping became the granddaddy of all fads in 1958 when baby boomers were kids. Now it's made a comeback among their hipster children and grandchildren.
They're doing it for fun, for choreographed performances and as a whimsical way to get fit and toned.
Actress Marisa Tomei began hula hooping a couple of years ago to train for her role as an erotic dancer in the film "The Wrestler." Last year, she released a fitness DVD called "HoopBody."
"It's so much more relaxed than going to a gym with people running on a treadmill," said Leigh Smith, 24, of Wildwood, Mo., between hoop classes at Koken Art Factory in the Fox Park neighborhood one evening. (She's not related to the instructor) "But the next day you really feel it in your muscles. You don't think it's a workout, but it definitely is."
Stefani Pollack, 34, of Webster Groves, Mo., had a baby nine months ago and said people often ask how she lost her pregnancy weight. Hula hooping, she tells them.
"I was attracted to it because the girls who do it are always smiling and don't look like they're hating themselves to lose five pounds," she added.
Smith reminds her students to whirl the hoops in the opposite direction around their waist once in a while to balance out muscle development in their abdomens.
Then she points to a woman whose hoop is swirling at an angle - higher in the front, lower in the back — and said the effect can be accomplished by leaning back slightly, looking at the ceiling and thrusting the hips upward.
Leigh Smith and Lena Myers giggle at the thought of this motion. They have a naughty description for it. Their friend Lindsay, who moved to Hawaii recently, introduced them to hula hooping, they say. She once pulled her collapsible hoop out at a party, snapped it together then showed off some tricks.
When Myers tells people she's going to hula hoop class they whirl their hips and ask if that's what she means.
"I tell them that I think it's a lot more than that," she said. "You're learning a skill as you work out."
Smith moves the class onto isolation tricks, which are done by holding the hoop away from the body.
She demonstrates a move called the warrior by flinging, twirling and twisting the hoop around in front and on the side of her body with her hands. Substitute nunchaku for the hoop, and Smith would look like a ninja.