SAO PAULO (AP) -- Eighth-grade teacher Ari Mascarenhas could have picked high-tech gadgets or modern apps to help his students learn Portuguese, but he instead went old school with the World Cup sticker book.
He's been a fan since 1986 -- when he was 8 -- and the attraction for the collectibles has trickled down generations and endured for adults who still trade the stickers in Brazil, the United States and other countries.
Mascarenhas said his soccer-loving students develop critical language skills by studying every part of the 80-page book filled with team rosters, country flags and historical info. They read stats, names and other information while associating it with colors, illustrations and other visual cues.
"With the sticker book they see that language goes beyond verbal. I loved the way they interacted swapping stickers, so I thought this year I could use an analog cue in this digital world," said Mascarenhas, a teacher at German-Brazilian Colegio Humboldt in Sao Paulo.
The book's popularity has spread as the World Cup nears its opening in Russia on June 14, despite sticker prices nearly doubling in some countries, including Brazil. That led to some grumbling in the South American nation, which has been in a financial crisis for the past three years with widespread poverty. Still, most of the 7 million sticker books put on the market quickly sold.
Panini, the Italian collectibles company that publishes the books, declined to say how profitable the World Cup books are, but the company itself had revenue of 631 million euros ($743 million U.S. dollars) in 2016, with products sold in more than 120 countries. Brazil is the largest market for the sticker books, followed by the United States with its sizable Latino population and England.
The allure for the books is similar to baseball or Pokemon cards -- challenging fans to complete the set. Each book has spots for 681 stickers depicting things like stadiums, players, host cities and the stickers themselves are sold in packs of six.
The days leading up to the tournament have become crunch time for collectors.
Actress Bruna Marquezine, the girlfriend of Brazil superstar Neymar, noticed high demand of stickers depicting the superstar. So she decided to swap stickers autographed by her boyfriend for those she still needs.
"I know this is cheating a little, but I will not have my sticker book incomplete this time," Marquezine joked on social media as she lured swappers.
Experts say fans would need to buy about 970 packs to fill their books without trades, because of the rarity of some of the stickers, though Panini CEO Mark Warsop said there's no difference in the frequency of stickers.
Mathematics professor Sebastiao de Amorim of the Universidade de Campinas said some stickers being hard to find is part of what makes collecting them enticing. Some are even sold at inflated prices.
"The minimum figure to complete the album is of 137 packages, but the odds of getting that, especially because some stickers are harder to find, are the same of winning the lottery," De Amorim said.
Panini is also hoping that a digital, mobile version of its paper product gains steam, like Pokemon and other titles that have proven popular in multiple formats.
Panini's sticker book app was downloaded more than 1.5 million times, introducing new ways to get stickers for users, including product placements. Warsop said he thinks it will pick up during future World Cups.
"The nice thing about the digital is that you can also swap and trade wherever you are, he said. "We want people to trade even if they are not in the same place."
Widespread use might be a ways off for adults who are currently introducing the hobby to kids.
"I can't sell my stickers there (on the app). People want paper," said salesman Renato Chaves, who took a van with more than 4,000 stickers to sell outside Brazil's training camp in Teresopolis, outside Rio de Janeiro.
Georgia Bulgackov, a 13-year-old student learning from the sticker book in Sao Paulo, said it's amazing to learn from a toy.
"What we love is to mix learning with something from our daily life, that made me understand more what the teacher wanted," she said.
Mascarenhas, her teacher, said he hopes his pupils can stretch their interaction with the sticker book to other parts of their lives.
"There are not many products that bring people together. In such a divisive world we can still swap, trade and have something in common," he said.