During science class last week at the Propel charter school in McKeesport, third-grader Curtis Harper discovered a new delicacy: frozen marshmallows.

"They are better cold and crunchy," Curtis said. "I'm going to go home and tell my mom to buy some big marshmallows and then I'm gonna put them in the freezer."

The taste test was part of a demonstration of how liquid nitrogen can freeze objects. It was one of a number of fun but educational lessons the Propel students learned during a community science fair sponsored by Bayer Corp.'s African American Employee Network.

During a morning and afternoon session, about 180 Propel students in grades three through six were able to make polymer gummy worms, Alka-Seltzer rockets and gak -- a substance similar to Silly Putty.

They also saw demonstrations of chemical reactions. They watched as liquid nitrogen froze balloons and tennis balls and, of course, the marshmallows that everyone got to taste.

Some were given a chance to try to put wooden skewers through balloons without popping them. Only one of the five students succeeded.

The trick to the exercise, as demonstrated by Bayer's Tom Barclay, was to put Vaseline on the end of the skewer and push it through the end of the balloon where the rubber -- the polymer -- hadn't been stretched.

Mr. Barclay is head of emergency response and security at Bayer, but his avocation is presenting entertaining science presentations to school children. Often throughout his lesson, he had the students giggling while they were learning scientific concepts.

In one exercise, he showed them a beaker filled with a liquid that he said was similar to that found in their teachers' stomachs. Then he added acid -- which he said is what's produced when the children get on their teachers' nerves -- and the liquid changed colors.

To return it to its original state, he added Alka-Seltzer to neutralize the acid.

He also showed them a chemical reaction that created foam that hardened into the substance used to create Nerf footballs.

After the demonstrations, the pupils lined up in groups to do three hands-on experiments themselves.

For the Alka-Seltzer rockets, they placed chunks of the drug into liquid in small capsules, placed a lid on them and then shook them twice. The capsules were then placed upside down in a tall, clear tower-like container, where they eventually exploded, sending the capsules flying to the top.

"I didn't think it was going to do that," said an astonished Kiara Simmons, 8.

Aaron Lockhart, Bayer's business development manager for wood and plastic coatings, explained that the mixture created a gas that caused pressure that "had to go somewhere," causing the small explosion.

When one of the capsules didn't pop, Mr. Lockhart discovered the container had a leak that's where the pressure was able to escape, he explained.

The students learned more about polymers as they made gummy worms, though not the edible kind.

For this exercise, they mixed sodium alginate with calcium chloride. When they poured the solution through a strainer, what was left in the strainer was a gooey polymer that formed a long worm-like object.

In a third exercise, the pupils learned about turning liquids to solids when they created gak, or play putty, from glue, water and borax, with a little food coloring added.

Mr. Lockhart said the point of the science fair is "taking science into the community and fostering the curiosity that kids have and directing it toward science."

The program offers areas "where kids normally would not be exposed" to the scientific concepts, he said. "Now that they know about science, they might turn out to be great scientists someday."

This is the fourth time Bayer has sponsored a community science fair. In past years, the fairs have been held at Aliquippa Middle School, the Homewood-Brushton YMCA and the Hill House Association.