NEW YORK (Reuters) - Trying to sell product to a 10-year-old? It takes a lot more these days than the prize hidden in the Cracker Jack popcorn or the maze on back of the breakfast cereal box.

In one of its most in-depth campaigns targeting youth, No.1 U.S. breakfast cereal maker Kellogg Co changed the shape of Apple Jacks cereal in a whodunit mystery beginning this week.

The strategy follows a number of companies who changed the stripes and spots of time-tested products and turned the eating experience into a game to attract media-savvy kids. Other recent gambits include Heinz's green tomato ketchup and a new promotion transforming brightly-colored M&Ms chocolate candies into black and white.

Mark Baynes, vice president of marketing at Kellogg, said such tactics were needed to make sure kids notice products amid a barrage of advertising and to help grow market share.

For Kellogg, putting more money into youth marketing is part of its strategy in a close race with No. 2 cereal maker General Mills, the maker of Cheerios and Wheaties. Kellogg last year conducted a mystery campaign to "find" the stolen colors of its Froot Loops cereal and has in the past changed the flavor of Apple Jacks.

In December, the Battle Creek, Michigan-based company raised its earnings forecast for fiscal 2003 and said it would use fourth-quarter tax savings to increase brand building investments.

Kellogg had already increased ad spending on its ready-to-eat cereal products from $216 million in 2002 to $227 million in the first 10 months of 2003, with children's marketing nearly one-quarter of the budget.

NEW GAMES, STORYLINES FOR PRODUCTS

Rachel Geller, chief strategic officer at youth marketing firm Geppetto Group, said more advertisers were adopting techniques of popular children's games that have several layers of reference and storylines, like the Pokemon trading cards and video games.

But she cautioned against crossing the line of a product's identity to the point it becomes unrecognizable, citing a cinnamon crunch twist on Kellogg's Frosted Flakes.

"If you always stay the same, you're going to get smaller and smaller. It's important to bring news to kids all the time ... and still make the product feel like it's familiar," Geller said. "But adding a chocolate flavor would be beyond the pale for Cheerios."

The new Apple Jacks commercials invite pint-sized consumers to find which scheming employee changed the cereal, first introduced in 1965, into X-shaped jacks. They can follow the trail of clues, from fingerprints to voicemail copies, on the Internet for the next few months.

"Apple Jacks is a weird concept ... because they don't taste like apples or look like jacks," Baynes said. "It's the kind of paradox kids love -- they like to get one up on the adults." The ads show a team of youths on the hunt for the perpetrators and probing the chemical properties of the new jacks. Publicis' Leo Burnett agency created the campaign, which recalls hit criminal forensics shows like "CSI."

Rachel Weiner, account director at Leo Burnett, said many kid-oriented marketers were looking at ad strategies using several layers of interaction.