Maybe you have gotten a CD-ROM from a cereal box. Or maybe you have played a soccer game online that the Nike company made. Or maybe you have played on, where you can follow the new, more grown-up Barbie and her friends.

Those things are all forms of advertising, but they are disguised as games. It's a combination that companies are calling "advergames."

Lots of big companies are using advergames: Pepsi, Mattel, Disney, General Mills, Kraft, Burger King, Nickelodeon.

Here's one way they work. Maybe this will sound familiar -- you may have played an advergame yourself.

Last summer when Nickelodeon wanted to draw more attention to the launch of its "Jimmy Neutron" series, it created the "Jimmy Neutron Gotta Blast" game. To play, kids needed a code from a Quaker Oats box to access the Web site and build their own rocket. To sweeten the offer, Nick promised that some of the rockets would be chosen at random to race on-air. That's called the "payoff." More than a half-million people played the game.

Advertisers like advergames better than TV commercials for several reasons. "People skip the commercials with their TiVo, they walk out of the room and miss the 30 seconds, there are so many channels," said Charlene Li, who studies how people use the Internet for a company called Forrester Research.

Li said most people who play a video game don't try to do anything else at the same time, so the game gets full attention. "And that's what marketers are interested in -- capturing their attention."

Here's the second thing advergames do well: They allow companies to collect information about the people who visit their Web sites. When you register to play a higher level of the game, or you fill out a survey, or you enter your score in a sweepstakes -- they get your age, city and e-mail address.

Advertisers are not allowed to collect information from kids under 13 because of the Child Online Protection Act. But they can collect it from your parents. If you are playing an advergame on the Hot Wheels Web site and want to sign up for the birthday club, for instance, parents must register and give their name, address, e-mail address and birthdates for themselves and their child.

And there's a third marketing benefit to advergames. If you like a game, you might e-mail it to a friend. That person gets the game, or a link to the game site -- and the attached ad. You are doing the work for the company.

Advergames can be made for much less money than it takes to produce a game for a PC or a console. While a PC game that might have 80 hours of play could take three years and $5 million to create, an advergame might take three to five months and cost between $20,000 and $250,000. Its play time might be an hour or less -- not great for a game, but pretty good for an advertisement.

Dave Madden is vice president of a company called Wild Tangent, which makes lots of advergames. He says advergames are one of the best ways to get a message out to kids.

"Kids don't read newspapers or magazines," he said, "but they are very savvy about the Internet. They're the hardest audience to please, so the games are just the best way to talk to them."

-- Ellen Edwards

The Spider-Man CD-ROM is free. But before you play, you have to watch a full-screen ad (shown here in inset box).Movie and TV standout "Jimmy Neutron" also is a hit on computer screens.