Trent Walking Elk built the battery-propelled pop bottle boat for one reason: "I thought it would make me win," said the fourth-grader. He was right. He won second place in the third- and fourh-grade computer-engineering category of the North Dakota Native American Science Fair on Monday. The fair was held in the ballroom of the Ramkota Inn.

Students from schools across the state competed in one of 11 to 16 categories, depending on the grade level. Most students competed at a regional and school level before arriving at the state competition, fair coordinator Darla Ratzak said.

Walking Elk's project was a popsicle stick propeller powered by a "D" size battery that was switched with a clothespin. The propeller started when a person pressed the clothespin together. His tri-fold poster showed photos of the boat moving through a water-filled bathtub.

The experiments covered several science disciplines, from chemistry to environmental science to math and physics and many more. Each student prepared a poster with photos and statements covering the scientific method: problem, hypothesis, prediction, test, data or results, and conclusion.

Some students explained scientific phenomena and others provided demonstrations. Some were standard science fair fare, with lemon circuits and shell-less eggs. Students were judged on their knowledge and the thoroughness of their project, as well as their application of the scientific method, more so than the originality of the project.

But originality did give some competitors an edge. White Elk Woman, an eighth-grader at White Shield, tested batteries and lighted a model earth lodge. She won first place in eighth-grade engineering.

"I chose the earthlodge because Iwanted it to fit in with my culture," she said.

She wanted to see what would be the best battery choice for powering the traditional home, she said.

Her model labels the parts of the lodge. Inside the lodge, she has three "D" size batteries. Each battery is attached to a light bulb.

She set up the model, with the battery-powered light bulbs in her father's office. Then she tested how long each battery lasted.

"The Energizer battery is best to use because it stayed lit for 40 hours," she said.

The students' projects were judged at least three times, and many were judged four or five times, Ratzak said.

At 1:30, students who placed first through third in each category in each grade level received a medal. Third and fourth grades were judged together.

The science fair was open to third through 12th grades. The students who place first in fifth through 12th grade qualify for the National American Indian Science and Engineering Fair March 23-25 in Albuquerque, N.M.

(Reach reporter Sara Kincaid at 250-8251 or sara.kincaid@;bismarcktribune.com.)