To keep students' attention, Purdue University professor William Chaney breaks up his 50-minute lectures on natural resources with a quick entertainment period he calls "Top of the Hour." One day it might feature a video of his daughter bungee jumping, another day a visit by his young granddaughter.

At Annapolis High School, science teacher Dan Pogonowski will stand on a desk if he thinks it will grab his teenage students. And Delores Zulian, a kindergarten teacher at Marumsco Hills Elementary School in Prince William County, changes activities frequently, letting her pupils "get their wiggles out" before they start poking their friends when they should be adding 2 plus 2.

From preschool to college, keeping students focused on a lesson for a sustained period is one of the toughest tasks educators face, and many say it is harder than ever. The factors that get in the way of concentrating in class include the many stresses of modern life, such as frenetic after-school schedules and even worries about terrorism.

Teachers and researchers say students can remain attentive and absorb a subject when they are interested in it and if they are presented with a mix of activities and discussion that breaks up the time. Students wholeheartedly agree.

"After about 15 or 20 minutes, you start to think about other stuff, zone out," said Jaclyn McLead, 18, a freshman in Chaney's class. "The Top of the Hour [break] keeps me going, before and after. I have six other classes, and no other teacher does this. It would be helpful if they did."

But many don't.

In college, the traditional lecture class -- often with hundreds of students -- is standard. With younger students, teachers say, it is increasingly difficult for them to experiment with time because they feel pressure to cover increasing amounts of material to meet content standards and help students pass high-stakes standardized tests.

"If I'm feeling pressed that I'm running out of time, to be honest, my activities will vary even less because I feel I have to present the material, and that's different from teaching the material," Pogonowski said.

Teachers face students with a wide range of attention spans, but much is made of the extreme cases. In fact, attention-deficit (hyperactivity) disorder is a separate issue. It is estimated to hinder the learning of at least one student in most classes, according to George DePaul, a Lehigh University psychology professor.

Though some experts say a normal attention span is three to five minutes per year of a child's age -- meaning that a 5-year-old should stay on task for at least 15 minutes -- there is no scientific consensus.

Scientists don't even use the term "attention span," said Steven Yantis, a professor of psychological and brain sciences at Johns Hopkins University. Research covers a range of brain processes that allow a person to focus on a single task and then stay focused, but more is needed to understand them, he said.

Multiple factors contribute to the ability to stay on task. It is not something hard-wired into the brain, said Patricia Carpenter, a professor at Carnegie Mellon University's Center for the Neural Basis of Cognition.

"Time of day, day of year, special events, age of child, weather, interest in subject matter, surroundings, diet, health, exercise, chemistry of the group, peer relationships, room temperature -- these are just some of the factors that impact attention," said Debra Van Dalen, a sixth-grade teacher at Greenville Middle School in Wisconsin.

But it is still hotly debated whether kids today have shorter attention spans than those of their parents' generation because of the rapid-fire television segments made famous by "Sesame Street," not to mention video games and MTV.

Though some teachers swear it is true, there is no proof, said Michael I. Posner, founding director of the Sackler Institute of Human Brain Development at Cornell Medical College in New York. To the contrary, scientists say, the brain functions differently for passive activities such as watching television than it does when working to absorb classroom lessons.

"We tend to blame an awful lot on cultural and environmental factors, and I think that is often a bum rap," said Ruth Spodak, a Rockville-based psychologist who specializes in learning disabilities, adding that many children labeled with attention-deficit (hyperactivity) disorder are misdiagnosed.

Still, teachers say short attention spans are an undeniable reality.

"Kids vary tremendously, but there are far more children who have shorter attention spans -- 15, 20 minutes or less -- than those who have longer ones," said Kathryn Baughman-Hill, a fourth-grade teacher at private Lowell School in the District.

"Look at the kind of images that TV presents: fast, exciting, extremely stimulating. Try competing with that on a daily basis -- it requires a lot of energy."

It's a lot to compete with at the college level, too. Tired of looking at blank faces, Chaney, a professor of tree physiology, created his popular break, figuring the time would have been lost to daydreaming anyway.

"It's too much to expect anybody to sit for 50 minutes straight and concentrate," he said. "The break also lets me personalize things. I create a better relationship with them, and that helps all of us."

For teachers, keeping students focused on the classroom lesson may be one of the most difficult parts of the job -- and it's getting harder all the time, educators say.