MUNCIE, Ind. -- Yo-yos seem to mesmerize generation after generation of America's youth. Yet, few youngsters realize they are learning about physics when doing tricks with the toy, say a pair of Ball State University educators.

When a youngster spins a yo-yo, hundreds of physics concepts are in motion, opening a vast new world for learning, according to James Watson, a physics professor, and his wife Nancy Watson, a science professor at Burris Laboratory School.

"For today's kids, a yo-yo represents something totally different from the computer games they are used to," Nancy Watson said. "There is this fascination when they first see the toy. For many students, it is the first time they've ever witnessed the motion. Some are totally in awe."

In the last few years, thousands of grade schoolers have been introduced to the yo-yo through a lesson plan written by the Watsons in conjunction with Mike Caffrey, director of sales and marketing at Duncan. The Ohio-based firm is one of the world's largest yo-yo producers and has a factory in Columbus, Ind.

In 1993, Caffrey asked the Watsons to develop the plan to spur the company's yo-yo sales nationally. The company anticipated when students see other children with the yo-yo on school playgrounds, many go to the nearest toy store to buy one.

About $80 million is spent by consumers on the toy in the United States with millions of yo-yos exported overseas. The yo-yo is one of this Christmas' most popular toys, according to several retail associations.

The Watsons have been using various children's toys to introduce scientific concepts to students for nearly three decades. Thousands of K-12 school teachers have attended their summer workshops at Ball State and at conferences around the nation.

Now known as the "yo-yo couple," the Watsons point out that America's fascination with the toy easily allows teachers to tap into young student's minds.

"These kits allow teachers to harness children, who really want to learn when they are playing with the yo-yo," James Watson said.

Grade-school teachers can buy kits of yo-yos, which usually retail for about $3, for $1. The kits also include historical background on the yo-yo, allowing teachers to use the toy as a interdisciplinary instrument.

"What we label 'playing' with grade school children is labeled 'experimenting' by doctoral level students in our physics classes," he said. "Both groups observe, hypothizing, testing and studying results. One group is playing while the other is studying the yo-yo. Both are having fun."

Students usually have the most fun during the final part of the lesson plan during a spinning contest, they said.

"The best part is six months later," Nancy Watson said. "I walk down the hallway and the kids are still playing with their yo-yos and teaching others about physics.

The Watsons claim the lesson plan has been a total success. Both have received hundreds of phone calls and E-mail messages requesting new lesson plans based on the yo-yo and other toys.