POCATELLO — Hot-air balloons the size of compact cars bobbled in the rafters of Idaho State University’s Holt Arena.

Near the 50-yard line below, American Falls middle schoolers gazed up, slack-jawed, hoping their balloons hovered long enough to beat the school record of six minutes and three seconds.

“We got four minutes and 15 seconds,” said eighth grader Gunner Giulio, after greeting his ballon on the turf. “Not bad.”

Science students at American Falls-based William Thomas Middle School participated in the school’s annual hot-air balloon launch Thursday. 

A little competition and field trip to the arena helps pique students’ interest near the end of the school year and after a round of standardized tests in the spring, said earth science teacher Gary Smith.

For 15 years, Smith has spearheaded the event, which emphasizes scientific principles like density, force and buoyancy.

“It also requires a ton of measurements and math,” Smith said.

Students work in groups and start by designing balloons, which take a number of shapes: balls, cubes, stars, cacti.

The kids then use heat-resistant mylar — “the same stuff Pop Tarts come in,” one student explained — to construct miniature models of their balloons. The mini hot-air balloons serve as the basis for the students’ larger, final designs.

Students construct the larger balloons by cutting out larger patches of mylar and taping them together.

On launch day, the kids help Smith and other teachers secure a hole at the bottom of each balloon over a stovepipe atop Camp Chef cookers at midfield.

In goes the hot air, up floats the balloon.

“It’s pretty cool,” said student Diana Harker.

In addition to grasping concepts like Archimedes’ principle of buoyant force, Harker said the project requires careful planning and precision.

To sustain a longer float time, students need to avoid puncturing the delicate mylar and tape air-tight seems. The better the construction, the longer the hang time.

Smith, who retires at the end of this school year, said he started doing the project on a smaller scale as a teacher in Texas. At the time, students constructed much smaller balloons out of tissue paper.

After moving to Idaho and glimpsing the Holt’s 90-foot-high roof, Smith saw room for a much bigger project.

“I said to myself, we’ve got to do it in there,” Smith said before watching another balloon ascend into the rafters.