Midway through a Friday afternoon physics class on apparent weightlessness, University of Pittsburgh lecturer Kiplin Perkins asks his students about a hypothetical scenario: If grass is grown on a turntable, would it grow a) angled out, b) angled in, or c) straight up?

He first poses the question verbally, then projects a diagram from a computer screen that gives students pictures of their three choices. With outstretched arms, students register their answers by clicking on individual devices that look like remote controls.

After all students "vote" their choice, a bar graph registers the distribution of their answers. Mr. Perkins reveals that the right answer is b, picked by about a third of the students.

He then removes two cardboard boxes to uncover little circular plots of grass, which have been growing (inward) on turntables for the last few days. A video camera zooms in on the grass and beams close-up images to the projector screen.

Let's just say that this isn't your grandfather's physics class. Or even your older brother's.

The remote controls, otherwise known as student response systems or clickers, are just one example of the many gadgets that have recently infiltrated college classrooms, both locally and nationally. Teachers are podcasting, giving quizzes online and posting their PowerPoint presentations on the Internet, to mention just a few technological tactics in play.

To some degree, teachers are trying to liven up lectures for a generation of students who might be able to rub their stomachs, pat their heads, listen to their iPods and instant message friends at the same time, but have trouble sitting still and focusing on the sound of one professor's voice for an hour and a half.

Like their movies and cell-phone ringtones, some students want their education "on-demand" -- and innovations like online quizzes and course outlines can give it to them.

But educational experts caution that while technology might be cutting-edge, it is not always the best educational model. Used without careful thought, even the most advanced technology might not beat pencil and paper.

Geri Gay, a Cornell University professor specializing in human-computer interaction, likens the use of technology in classrooms now to the beginnings of television as a tool for children's education.

She said that the revolutionary children's show Sesame Street was successful not because nobody had ever thought to put puppets on television, but because its founders spent countless hours researching how children learn. Big Bird and Cookie Monster, therefore, were deployed strategically to accomplish specific missions.

"The bottom line is that you really need to structure it," she said. "You need to be thoughtful about why is it there and what are we trying to do."

As a cautionary tale, Dr. Gay describes one experiment in which 250 Cornell students carried laptops that researchers could track by location. When a professor of an early morning computer class decided to post the notes to his lectures online, researchers noted that the laptops of students in the class (and presumably the students themselves) often stayed in their dorm rooms, rather than appearing in class.

The researchers, who could record which Web sites students were visiting, also noticed a spike in access to the online notes just before the exam. That professor, said Dr. Gay, continues to post his lecture notes online, but has also incorporated attendance into his grading system.

Indeed, a dropoff in attendance seems to be the most likely casualty of one of the most common technological techniques: posting material from classes on the Web.

Mr. Perkins, the physics lecturer at Pitt, doesn't post lecture notes online. "If I were to post them," he said, "some students might feel they didn't have to come to class."

It was an issue that Duke economics professor Lori Leachman was well aware of when she decided to podcast her classes, which she did for the first time in fall of 2004. In this context, podcasting simply meant recording her voice during the lectures and later posting the recording online as an audio file.

But rather than posting the lectures during -- or immediately after -- class, Dr. Leachman waited a few weeks and posted them in bunches. "I teach at 8:30 in the morning and I didn't want to disincentivize coming to class," she said.

Podcasting is still relatively rare and experimental on college campuses. This semester, Dr. Leachman is stretching the technology even more: Any slides or graphs that she puts up during classes will be included as a visual component alongside the podcast.

Unlike podcasting, the development of the "clickers" has actually been progressing for decades in the field of physics.

In the early 1990s, researchers developed a test called the Force Concept Inventory that demonstrated to many physicists that traditional lecture classes were simply not an effective way of teaching most concepts.

The FCI asked students tricky questions that pitted intuitive beliefs against concepts in Newtonian physics. At the high school and college level, teachers were dismayed to see most of their students fell for intuition, rather than science.

Teachers found that when they stopped their classes to do interactive exercises, the scores went up. They tried gauging student opinion by a show of hands, or holding up signs, but the clickers seem to be the perfect tool.

"The clickers are the sort of technological innovation that lets everybody respond," said physics professor Peter Koehler. "The system records which student answers which way, so I know, and they know I know, but among themselves they are anonymous."

Dr. Koehler also uses the devices to take attendance, and gives students a slight bump in their grades when they answer questions correctly on their clickers.

He said that the devices not only force students to think about the material, but also let professors know when students are not understanding a concept.

"What I've learned is that some things that I thought I made perfectly clear, I didn't," said Dr. Koehler. "Learning that and knowing that right there instead of at the exam gives me a chance to correct it."

At Penn State-New Kensington, history professor Joseph Coohill is giving his students a chance to show him what they know -- outside of class time rather than inside. His students take their quizzes online.

Dr. Coohill likes the system because it doesn't take up class time, and students like it because they can schedule the quizzes anytime and anywhere they can get an Internet connection, be it in their dorm room in the middle of the night or over breakfast at a coffee shop.

But as much as he likes the quizzes, Dr. Coohill is not just skeptical, but downright disdainful toward much of the other instructional technology.

He refuses to use PowerPoint presentations, calling them "anti-intellectual garbage" and "junk." And he will not put course outlines online, something he sees mainly as an incentive for students to skip class.

While he's enthusiastic about the administrative headaches that computers have eliminated -- no more overhead projectors, films or cumbersome photocopying of documents -- he's not yet convinced about technology's true educational benefits.

"I've been using Blackboard for five years," he said, referring to a popular brand of class management software, "and I'm still not convinced that it makes my history classes better. They're more efficient and more organized, but whether they're learning more, I don't know."