For toymakers trying to invent a new plaything for Christmas, this is the stuff of dreams:

In July, Mattel shipped a strange and gooey new play substance called Gak.

Last month, its popularity had gotten so out of control at Prince William County's Linton Hall, a school run by Benedictine nuns, that it was banned.

"The kids were stretching it from one corner of the room to the other, over each other's heads. They were bouncing it and throwing it. Every child was going for it," said second-grade teacher Angie Ashley, who imposed the Gak-out. "It gets cold. It feels weird. They wouldn't put it away. It was getting so that homeroom wasn't a real good experience."

In a season when retailers have pushed few new toys, Nickelodeon, MTV's network for children, has spawned an inexpensive product that's a breakthrough of sorts: one of the first TV spinoffs not somehow tied to a character, the way Sesame Street dolls are tied to, say, Ernie. With its help, prepubescents are returning to that oddly primal American cultural tradition: a substance that freaks out Mom.

"Bart Simpson would like this," said Istar Schwager, an educational psychologist, of the new compound with the consistency of raw liver. Even Peggy Charren, founder of the watchdog agency Action for Children's Television, is sanguine about the TV spinoff. "It's a better mess than most programs on television," she said.

The jiggly, gross, stretchy, slippery, icky, and strangely addictive substance smells like the butt end of a bad bottle of wine, feels like oysters, and comes in nine strikingly bright colors.

Wal-Mart sold more than 160,000 units in six weeks at $3 a pop. Gak is outselling Silly Putty, Turtle Ooze, the Batman figures and Crash Dummies, according to Mattel.

What goes into the creation of such a new icon of play reveals a great deal about recent American history, psychology and capitalism.

Making castles out of sand -- not to mention pies out of mud -- doubtless predates human time, as does flinging carrot puree at one's sister. But nobody made serious money off weird goop until 1950. That's when Peter Hodgson invested $147 in a failed rubber substitute that he christened Silly Putty. When Hodgson died in 1976, he left an estate of $140 million.

Play-Doh and finger paints also were sensuous staples of the Baby Boom's childhood. But those were basically craft materials. Only recently have toymakers focused on goop for its own sake.

In hindsight, the market potential was obvious. Kids "are just discovering their sensory systems," noted James C. McNeal, a behavioral scientist who has studied play for 25 years. "It doesn't matter if it is rough or smooth. They just simply have a strong sentience need."

Toy mavens believe the first product specifically capitalizing on this was Slime.

In 1982, producer Roger Price cooked up a show for Nickelodeon called "You Can't Do That on Television." The plot was that the program's owner was so cheap he wouldn't hire adults. So he hired kids. Then, every time his little violations of the child-labor laws said "I don't know," green Slime would come dumping on their heads.

"The Slime on that show became a symbol of the solidarity of oppressed kids," said Scott Webb, creative director of Nickelodeon. "It's tough to be a kid in the adult world. They don't get cut any slack. When they don't know how to do something right, they get slimed. We never use mess to humiliate kids. It's a celebration of how tough it is to be a kid."

The show was a hit. Coincidentally, chemists at Mattel had begun to look for a product that would have the "delight factors" of being green and drippy, but would be shelf-stable and nontoxic.

Play-Doh-like substances are made of flour and a drippy gooey binder. The "Eureka!" moment on Slime came when the chemists realized the binder substance -- all by itself -- was marketable. Thus was Slime born.

It was a hit. With Mattel manufacturing Slime, and Nickelodeon, in effect, promoting it, 12 million units were sold in the first six years. But no relationship existed between the firms until two years ago, when Mattel slapped the Nickelodeon name on its packaging and Nickelodeon began to receive a percentage of sales. Neither side will say how much.

Nickelodeon, in the meantime, had developed "Double Dare," a game show in which contestants caught raw eggs with cymbals and dived through giant sundaes and reached up into huge nostrils to retrieve flags. The various kinds of messes were christened, generically, "gak."

Corporate relationship now established, Mattel chemists began to search for a new! improved! icky substance that could be marketed as Gak. In two months, they created a substance that was stretchy, could be made in a variety of bright colors, and had a cold, wet feeling.

Mattel prefers to be mysterious about that most-asked Gak question -- "What is this stuff, anyway?" However, Keith Flohr, manager of analytic services at Artech Corp. in Chantilly, a testing laboratory, analyzed a sample at the request of The Washington Post.

Flohr believes Gak is an ingenious array of acrylic and silicone with a few odds and ends like clay thrown in.

The acrylic is a thickener. It also is used to hold the moisture inside a disposable diaper. It is indigestible, Flohr says, so that when tykes inevitably nibble at it, it goes right through them. Also, mold and bacteria won't grow on it.

The silicone, of which breast implants are made, would make Gak a half-way decent window putty, he says. It is airtight even when stretched. That is why when Gak is pushed into its Splat container, it makes marvelously rude noises. It also allows Mattel to market a separate toy that inflates Gak into a bubble the size of a kid's head.

Gak's unnerving feel -- it seems moist, but leaves little wetness on the hand -- is created by the hygroscopic properties of calcium phosphate, says Flohr. Gak achieves a moisture balance comparable to a human tongue made dry by a person breathing through his mouth. If you touch this tongue, it will feel wet. But it is sufficiently dry as to leave very little moisture on the hand.

A Mattel spokesman refused to confirm or deny Flohr's findings. But John Handy, who spearheaded the Gak team, admitted that Gak contained a little glycerin. That's to make mommy's hands feel softer after she plays with it, hopefully lessening her resistance to the product. Gak also contains germicidal agents, to make sure that kids don't spread their colds through the stuff.

But this still doesn't explain its strangely addictive quality.

When Mattel watched people through two-way mirrors, they noticed that both kids and adults would not leave Gak alone. "One mother was going on about how she found this stuff repulsive," remembered Handy. "After about 20 minutes of this, another parent asked her whether she realized that during her entire speech, she had been playing with the Gak nonstop."

That makes a lot of sense to psychologists who point out that Silly Putty has long been a stress-reliever for Wall Street traders and smoke-ending programs.

Another thing researchers think is significant: Girls play with Gak just like boys. This is not frequently the case with toys. Boys will start throwing their toys around early in play, unlike girls. Gak, however, led to leaping, screeching, tossing-around play equally quick with both sexes.

What's next? Will this new delving into the ids of kids produce an endless progression of weirder and more sensuous substances?

Nickelodeon, which now reaches into 54.7 million American homes, has no show in the works built around a new kind of mess.

That is why the toymakers are speculating about a new market for Gak -- the adult market. They notice that around their own offices, adults play with it while they talk on the phone. They toss it across the office.

"Being a kid isn't about how old you are," said Nickelodeon's Webb, himself 33. "It's a point of view. It's how willing you are to play."