By Neal Singer The 200 children sitting on the hardwood floor watch as physics professor Mats Selen claps two chalkboard erasers. Chalk dust flies up into the darkened gymnasium at Martin Luther King Jr. Elementary School in Urbana. Above Selen's head, the dust makes visible a conventional flashlight beam as it spreads and disperses weakly. The dust also makes evident a pencil-thin beam that does not disperse at all as it crosses the entire gymnasium above the children's heads. That beam - made up of the coordinated light waves that emerge from a laser - sparkles ruby red. They stare upward, momentarily silent. "Lasers are useful," says Selen through the darkness, "for holograms, gun sights and surgery." Later, he pulls a silken tablecloth from under a table full of dishes and no dish falls. The children make sounds of disappointment. "Inertia," says Selen. "Don't try this at home." In another demonstration, undergraduate physics students Gregg Thayer and Greg Rudnick play the roles of two electric charges. Wearing the same color hats, they flee to opposite sides of the room and make faces at each other. But donning hats of different colors, the two students run longingly toward each other, arms outstretched in exaggerated pantomime, and hug. The children laugh. "Charges that are the same, repel; opposite ones attract," says UI physics senior Elizabeth Vokurka, who activates a chemical-propellant fire extinguisher as she sits on a cart. The cart rolls in the opposite direction from the direction the propellant is aimed. "Action and reaction," she tells the children as she rolls backward. "Like a rocket in space." "Science is fun," says Selen at the end of the half-hour performance, mostly led by university students. "That's what I want you to remember. Science is fun, and I get to do it all day long. It's my job." The program, designed to motivate grade school children to consider science as a career, is led by Selen. He began the effort early this year after a discussion with UI physicist Inga Karliner, who is leading the department's three-pronged physics outreach program. This includes Saturday morning physics lectures on campus for high school honor students, arranged by physicist David Hertzog and office manager Penny Sigler, and a Saturday teacher's workshop organized by Karliner. "Teachers who want to teach science don't have a lot of resources," said Selen. "And people who teach kids may not be scientists and even may be intimidated by science. "Unless you have an exceptionally good elementary school science teacher, kids may feel that science is hard, boring or beyond their grasp," he said. "The point of going out to schools is to show that normal people just like them can do this stuff." Selen and his 10 undergraduate helpers rent a van from the UI motor pool to take them to each performance, initially being held in grade schools within an hour's drive of Champaign-Urbana. The group also hopes to perform science in day camps this summer, and a Paris, Ill., school has inquired about being on the itinerary. "Word is diffusing," said Selen, and then, correcting himself to sound like an ordinary person, "I mean spreading." The Saturday morning physics programs for high school students will conclude this semester with a talk May 7 by UI scientist Paul Lauterbur, "Magnetic Resonance Imaging: From Physics to Medicine," in 1005 Beckman Institute. Other talks have included, "Looking Into the Brain With a LASER," by Enrico Gratton; "Liquid Crystals: Strange Fluids That Don't Always Flow," by Paul Goldbart; "How Cold is Cold and How Cold Does It Have to Get to Levitate a Train?" by Don Ginsberg; "What is Everything Made Of?" by Gordon Baym; and "The Particle Zoo and Who's Behind the Bars," by Tony Liss. Baym's recent lecture made clear why certain physicists around the world are interested in building big circular tunnels in which to smash particles to imitate, in a limited way, the activity some scientists believe took place during the creation of the universe. "The advantage of having scientists meeting with students is that when a scientist teaches, there is no end of the depth of the questions kids can ask," Karliner said. She describes watching Larry Smarr, director of the UI's National Center for Supercomputing Applications, make science come alive for grade school students by showing them a videotape taken by a satellite as it flew over the planet Venus. Karliner's physics workshops for teachers have included a particle physics lecture by theorist Scott Willenbrock; a description of the program "Operation Physics," developed by the American Physical Society to train master teachers; a demonstration of Logowriter Robotics with Legos; and an opportunity to conduct physics experiments. The workshop is a pilot program intended to give the physics department an indication of teachers' needs that the department can meet. Other science outreach programs from campus are in place in biotechnology, NCSA, chemistry, the life sciences and for minority groups.