It takes guts for a high school student to tell a Boston University scientist that his test tube's not quite full.

Yet Shantanu Gaur essentially did that when he challenged his mentor's conclusions about retinal development in chick embryos.

The senior at Bethel Park High School conducted his own research, disproving his mentor and winning a top spot yesterday in the 65th annual Pittsburgh Regional Science and Engineering Fair.

In all, 700 students from 90 schools in 12 counties participated in the two-day event at Heinz Field.

"Some of the competition is just brutal," said Charles J. Vukotich Jr., chairman of SciTech Festival and assistant deputy director of the Allegheny County Health Department. The science fair is part of the festival; remaining activities will be held April 17-25 at the Carnegie Science Center and other locations.

A walk around the science fair exhibits would have reassured those who lament the amount of time kids spend eating junk food and playing video games.

Aside from Harvard-bound Gaur's work on chick retinas, students tackled such topics as stem-cell research, the effectiveness of aspirin vs. ibuprofen, bacteria growth in water bottles, stream quality and carbon sequestration.

Humor came across in such projects as "Breed or not to breed" and "Testing submarine shapes for the hull of it." And behavioral science was amply represented, too, with one student testing how sleep duration might affect an 8-year-old sibling's behavior.

Some of the work was described as ground-breaking. "These are high school students doing Ph.D.-level work," Vukotich said.

The fair offered first-, second-, and third-place awards in 20 categories, from biology to robotics to medicine. Students competed in one of three age groups, with older contestants having the opportunity to win not only bragging rights but college scholarships.

Gaur and two others -- Di Ye from Mt. Lebanon High School and Lekha Tummalapalli from Fox Chapel Area High School -- were selected to participate at the Intel International Science and Engineering Fair in Portland, Ore., next month.

Described as the "Olympics, the World Cup and the World Series of science competitions," ISEF gives 1,200 students from 40 nations a chance to compete for scholarships, scientific field trips and a grand-prize trip to the Nobel Prize ceremonies in Stockholm, Sweden.

Gaur won third place in an ISEF event last spring. He spent the summer at Massachusetts Institute of Technology, working under the supervision of Boston University researcher Robert Hausman.

He said Hausman believed the inhibitor DTNB affected retinal development in chick embryos by attaching to the enzyme R-cognin. But Gaur, who plans to pursue a medical degree and a doctorate, sought another explanation for why the beneficial enzyme ceased functioning.

He said his research showed DTNB attaches not to the enzyme but to a receptor on the embryo cell. In other words, the enzyme and inhibitor vie for the same "dock" on a cell.

Gaur said his research may prove useful to the study of macular degeneration and glaucoma, diseases characterized by a weakening of the retina. Perhaps, he said, an R-cognin deficiency is involved.

Tummalapalli, a junior, did her research on the use of fetal liver cells to treat patients with liver disease. She said she and her mentor, Satdarshan P.S. Monga of the McGowan Institute for Regenerative Medicine at the University of Pittsburgh, have discovered fetal cells have advantages over the use of adult cells when used as a filter in a bio-reactor, a cutting-edge dialysis-style machine for the liver.

Ye, a sophomore, studied carbon sequestration. Laymen might take a cue from her project's subtitle, "Turning lemons into lemonade."

She said she found support for the hypothesis that bauxite residue, an alkaline by-product of aluminum production and possible cause of groundwater pollution, can be neutralized with carbon dioxide and water.

At the same time, the interaction may help protect the ozone layer. Ye said bauxite traps the carbon dioxide, removing a greenhouse gas from the air.

"I see there's a future in this idea," she said.