Like clockwork at 3:15 p.m., the eighth grader - Scotch plaid shoes, cell phone pressed to her ear - exits Tairina Grocery near the corner of Fourth and York, her first stop out of Welsh Elementary School, 30 seconds away, one of a steady stream of kids leaving the store with filmy black sacks.
The contents of hers is typical - two one-ounce bags of Herr's Salt & Vinegar chips, a can of Coke, and a cake called Elim's Delight, favored by North Philadelphia's schoolkids for its price point: 25 cents.
This ritual - often played out before and after school - adds an average of 360 calories (per visit) to the kids' daily total, subverting the laborious fine-tuning of school lunches, and upping the odds of obesity-related disease.
No one is foolish enough to think they can put an end to it: The after-school snack has achieved the irreversible status of, well, a constitutional right.
But with an astounding half of the elementary-school students in many of the city's poorer African American and Latino neighborhoods now overweight, and with an equal number of them regularly visiting the corner stores that lure them with chips and cake, there is what nutritionists and obesity researchers see as, well, a window of opportunity.
One of those researchers is Gary Foster, head of Temple's Center for Obesity Research and Education. He and his team joined the nonprofit Food Trust's nutritionist Sandy Sherman and her field workers one day last week before the youngster from Welsh and her classmates hit the stores.
"It's a big behavioral target for us," Foster said. "If it was 100 calories [a visit], it would be hard to reduce. But at 360 calories a visit, you don't need to change that much - cut back to a single serving. . . . get them to buy a water instead of Hugs," the colored sugar water in little barrel-shaped bottles.
Knock off just 50 calories, he says, and you get closer to the number of calories the kids are expending, closer to closing what he calls "the energy gap."
But obesity research has been notoriously flimsy, based on suspect self-reporting, and suppositions about access to certain foods.
So to test the proposition, a dozen corner stores around five city schools (besides Welsh, Clymer, Fairhill, Kenderton, and Robert Morris) are being not only studied, but manipulated: They're asked to post the Food Trust's Snackin' Fresh posters ("Small Size, Big Taste!" is one), stock bottled water, and visibly display coolers of chopped-fruit salads.
At Diogenes Grocery at Lawrence and York, owner Diogenes Luciano is an early-adapter: Last September he was making 100 clamshells of chopped fruit and distributing them to 40 corner stores; last week he was up to 800.
(Five other public schools are being used as a control group, the inventories and lay-outs of nearby stores unchanged; but the schools' student bodies, as it were, measured just as rigorously.)
To get a handle on what kids are really buying, Temple's center has deployed staffers with clipboards who stand on the stores' doorsteps, asking kids if they can take a look at the contents of their bags.
It was staffer Cornell Davis, a pleasant young man in a jeans jacket, who "intercepted," in the academic lingo, the eighth grader who'd just spent $1.80 on her haul. If she'd consumed one of the bags of chips, the chocolate cake, and the Coke, she'd have packed in an extra 500 calories, assuming that was her only corner-store stop of the day. (About 38 percent of the surveyed students said they stopped in twice a day.)
By comparison, Welsh's school lunch that day was a choice of French bread pizza or "beef patty on a bun" (also known as a hamburger), baby carrots and ranch dressing, blended fruit juice, and 1 percent milk.
A student can pick three of those items; that lunch is meticulously calibrated (down to the milligrams of Vitamin C and calcium) to average out at 504 calories - the same amount as the young girl's bag of snacks, and a stark reminder of the real-world gap.
Philadelphia recently won dispensation to continue its federally supported "universal feeding program," ensuring free meals for students in poorer neighborhoods without requiring formal signups.
Public school hallways have been stripped, for the most part, of vending machines filled with soda and sugary snacks.
And in the classroom, bilingual Food Trust educators such as Mara Castro preach the gospel of smaller portions and smarter choices.
But when the bell rings, there are still the dim corner stores - doors open, walls of chips beckoning, cakes and pies conveniently at eye level.
The Food Trust's brightly labeled bottled water languishes in their coolers while sugary mini-Hugs fly off the shelf.
No mystery on that front: The water is priced at 50 cents, the Hugs at 25, leaving another quarter, if you're so inclined, for a cake called Elim's Delight.