For the past 10 years, companies that come up with a cool idea like Yu-Gi-Oh have made a lot of money by using that idea in many different products, TV shows, and movies for kids.
By spreading the idea around, companies make it seem impossible for kids not to buy something related to the fad.
There's nothing wrong with buying these products. But kid-entertainment experts say kids should be more aware of how companies get them interested in products. Kids also need to know why companies make these products.
"It's about getting you to spend your allowance money on these things," says Kathryn Montgomery, president of the Center for Media Education in Washington, D.C.
The Yu-Gi-Oh brand is a good example of how a good idea becomes a fad.
Yu-Gi-Oh began as a comic book in Japan in 1996. The character Yu-Gi is a kid in high school whose grandfather owns a game store. The grandfather gives Yu-Gi an ancient Egyptian puzzle.
When Yu-Gi solves the puzzle he receives magic powers. One of his talents is playing Dueling Monsters - a card game in which two players battle each other using cards based on mystical creatures with different abilities.
The comic book was a huge hit in Japan, partly because it involved trading cards, which many kids were playing.
The company that owns the Yu-Gi-Oh idea in Japan has made more than $2 billion. Because other products that did well in Japan have become big successes in the US (Power Rangers, "virtual pets," Nintendo, etc.) the company sold the idea to the same US company that produced Pokémon - 4Kids Entertainment.
Even before the cartoon went on TV in the fall of 2001, 4Kids was slowly introducing it. "We had to tell kids who [Yu-Gi] was, how the game was played, what various power levels the monsters had, and who his circle of friends were," says Steve Syatt, a 4Kids spokesman.