Maurice A. Scales worried about smashing his little sister's fingers in the door. Christina Ahrens wanted to help boys keep their zippers up. And Michael Bysiek was tired of wasting time in the bathroom.

The three young pupils invented contraptions to solve their problems, and yesterday they were among 45 regional winners honored in the national "Invent America" contest. Nine inventions, one each for grades kindergarten through eight, will be chosen this week and will be displayed throughout the summer at the Smithsonian Institution Museum of Natural History.

At a colorful celebration in Western Plaza, on Pennsylvania Avenue between 13th and 14th streets NW, hundreds of parents, teachers, tourists and passers-by gathered to watch the inventors be presented awards by Dick Rutan and Jeana Yeager, copilots of the Voyager on the first nonstop, non- refueled flight around the world.

Under towering red, white and blue balloon sculptures, two bands played and booths offered souvenirs, snacks and much-needed Kool-Aid. The winning youths, clad in Hawaiian leis and Invent America T-shirts, stood on stage in the sweltering heat waving tiny American flags and squinting while photographers snapped pictures.

"Smile, Zevie," yelled Sarina Steinmetz, proud parent of Zev Steinmetz from Akiva Hebrew School in Oak Park, Mich. Zev Steinmetz, in blue shorts and a yarmulke, was pouting. But the 6-year-old inventor of a plastic device to prevent a Popsicle from dripping onto fingers grinned when his mother made a silly face.

His "Drip Guard" was on display under a blue and white tent with other gadgets, including a self-cleaning bathtub, a shoe with springs on the bottom to improve leaping ability, the "Cat Off Aquarium Topper" (a sloped cover for an aquarium), and red salt to allow better monitoring of sodium intake.

They were created by youngsters such as 5-year-old Ruby Lopez, who had traveled with her mother from Clovis, N.M., to show off a device for quickly finding a rubber band to wrap around her ponytails.

"I thought of it in my head. I've never seen one in a store," Ruby said of her "Quick and Easy Rubber Bands." Ruby had stretched a clothes hanger "till it looked like a long stick" and wrapped several rubber bands around it. "It hangs in your closet, and when your mom is going to comb your hair you just pull off the rubber band."

Christina Ahrens, 12, of Tempe, Ariz., was not thinking of her wardrobe when she invented the "Fly Guard." Christina's gadget, when attached to the pants zipper of a boy at school, would trigger a warning buzzer if the zipper starts to fall.

Nearby, Jeffrey Wong was exhibiting a slightly more sophisticated device: a portable heart de-fibrillator. The 13-year-old from Freehold, N.J., said "Shocker II" uses electrical current to stop fibrillation, a rapid series of contractions of the heart muscle causing irregular heartbeat.

Rutan, copilot of the record-breaking December flight of the Voyager, had just flown in from the Paris Air Show.

"The thing that bothers me a lot about America is that people limit themselves," he said. "One guy here wants to be a pilot. I told him that he ought to be thinking about buying the airline or creating his own company."

About 10,000 schools participated in the corporation-sponsored program, administered by the U.S. Patent Model Foundation.

All 45 regional winners received $500 U.S. Savings bonds, and the nine national champions will get additional $500 bonds. The winners will be announced Thursday.

Many of the contraptions were born of necessity.

Mary Margaret Petruzzo dreamed up her "Foldamatic" when she grew tired of folding napkins for the dinner table. Now, when she lays a paper napkin on top of a floral cardboard box, it falls into a plastic gadget and can be folded a split-second faster.

"I thought of this so it wouldn't take so long," said Mary Margaret, 10, of the Grace Christian School in Bowie.

"Baby No-Mash" was inspired by the accidents of Maurice Scales' little sister, Amaleya.

"My sister's fingers always got squeezed in the door," said Maurice, 7, of Suitland. His Plexiglas device hangs on a door and keeps it open a few inches.

Like all creative thinkers, these young inventors often drew their inspiration from odd places.

Take Michael Bysiek and his "Electric Toilet Paper" device that, with the touch of a button, dispenses a roll of toilet paper so he does not have to turn the roll himself.

"My grandfather has a radio attached to his toilet paper dispenser," explained Michael, 6, from Olean, N.Y.

"I sat in my grandfather's bathroom for a long, long time looking at his musical toilet paper, and I thought, 'Wouldn't it be a good idea to push a button and have the toilet paper just come out?' "