BEIJING (NYTIMES) - Li Chengzhi had a lot to learn when he first got a job as a professional censor.
Like many young people in China, the 24-year-old recent college graduate knew little about the 1989 Tiananmen Square crackdown. He had never heard of China's most famous dissident, Liu Xiaobo, the Nobel Peace Prize laureate who died in custody two years ago.
Now, after training, he knows what to look for - and what to block. He spends his hours scanning online content on behalf of Chinese media companies looking for anything that will provoke the government's wrath.
He knows how to spot code words that obliquely refer to Chinese leaders and scandals, or the memes that touch on subjects the Chinese government doesn't want people to read about.
Mr Li, who still has traces of youthful acne on his face, takes his job seriously. "It helps cleanse the online environment," he said.
For Chinese companies, staying on the safe side of government censors is a matter of life and death. Adding to the burden, the authorities demand that companies censor themselves, spurring them to hire thousands of people to police content.
That in turn has created a growing and lucrative new industry: censorship factories.
Mr Li works for Beyondsoft, a Beijing-based tech services company that, among other businesses, takes on the censorship burden for other companies. He works in its office in the city of Chengdu.
In the heart of a high-tech industrial area, the space is bright and new enough that it resembles the offices of well-funded start-ups in tech centres like Beijing and Shenzhen.
It recently moved to the space because customers complained that its previous office was too cramped to allow employees to do their best work.
"Missing one beat could cause a serious political mistake," said Mr Yang Xiao, head of Beyondsoft's Internet service business, including content reviewing. Beyondsoft has declined to disclose which Chinese media or online companies it works for, citing confidentiality.
China has built the world's most extensive and sophisticated online censorship system. It grew even stronger under President Xi Jinping, who wants the Internet to play a greater role in strengthening the Communist Party's hold on society.
More content is considered sensitive. Punishments are getting more severe.
Once circumspect about its controls, China now preaches a vision of a government-supervised Internet that has surprising resonance in other countries. Even traditional bastions of free expression like Western Europe and the United States are considering their own digital limits.
Platforms like Facebook and YouTube have said that they would hire thousands more people to better keep a handle on their content.
Workers like Mr Li show the extremes of that approach - one that controls what more than 800 million Internet users in China see every day.
Beyondsoft employs over 4,000 workers like Mr Li at its content reviewing factories. That is up from about 200 in 2016. They review and censor content day and night.
"We're the Foxconn in the data industry," said Mr Yang, comparing his firm to the biggest contract manufacturer that makes iPhones and other products for Apple.
Many online media companies have their own internal content review teams, sometimes numbering in the thousands. They are exploring ways to get artificial intelligence to do the work.
The head of the AI lab at a major online media company, who asked for anonymity because the subject is sensitive, said the company had 120 machine learning models.
But success is spotty. Users can easily fool algorithms.
"The AI machines are intelligent but they aren't as clever as human brains," Mr Li said. "They miss a lot of things when reviewing content."
Beyondsoft has a team of 160 people in Chengdu working four shifts a day to review potentially politically sensitive content on a news aggregating app.
For the same app, Beyondsoft has another team in the western city of Xi'an reviewing potentially vulgar or profane content. Like the rest of the world, China's Internet is rife with pornography and other material that many users might find offensive.
In the Chengdu office, workers must put their smartphones in hallway lockers. They can't take screenshots or send any information from their computers.
The workers are almost all college graduates in their 20s. They are often unaware of, or indifferent to, politics. In China, many parents and teachers tell the young that caring about politics leads only to trouble.
To overcome that, Mr Yang and his colleagues developed a sophisticated training system.
New hires start with weeklong "theory" training, during which senior employees teach them the sensitive information that they didn't know before.
"My office is next to the big training room," Mr Yang said. "I often hear the surprised sounds of 'Ah, ah, ah'... They didn't know things like June 4," he added, referring to the 1989 Tiananmen Square crackdown. "They really didn't know."
Beyondsoft has developed an extensive database based on such information that Mr Yang calls one of its "core competencies". It also uses anti-censorship software to regularly visit what it calls anti-revolutionary websites that are blocked by the Chinese government. It then updates the database.
New employees study the database much like preparing for college entrance exams. After two weeks, they have to pass a test.
The screensaver on each computer is the same: photos and names of current and past members of the Politburo Standing Committee, the Communist Party's top leadership.
Workers must memorise those faces: Only government-owned websites and specially approved political blogs - a group on what's called a white list - are allowed to post photos of top leaders.
Workers are briefed at the beginning of their shift on the newest censoring instructions sent by clients, which the clients themselves receive from government censors. Workers then must answer about 10 questions designed to test their memory. The results of the exam affect the workers' pay.
One question on a recent Friday: Which one of the following names is the daughter of Li Peng, China's former premier? The correct answer is Li Xiaolin, a longtime target of online ridicule for her expensive fashion taste and for being one of many children of senior officials who come into high positions or wealth.
That's a relatively easy one. A tougher test is parsing the roundabout ways that China's Internet users evade stringent censorship to talk about current affairs.
Take, for example, a Hong Kong news site's 2017 commentary that compared the six Chinese leaders since Mao Zedong to emperors during the Han dynasty. Some Chinese users started using the emperors' names when referring to the leaders.
Beyondsoft's workers have to know which emperor's name is associated with which leader.
Then there are the photos of an empty chair. They refer to Liu, the Nobel laureate, who wasn't allowed to leave China to attend the award ceremony and was represented by an empty chair. References to George Orwell's novel 1984 are also forbidden.
Beyondsoft's software trawls through webpages and marks potentially offensive words in different colours. If a page is full of colour-coded words, it usually requires a closer look, according to the executives. If there are only one or two, it's pretty safe to let it pass.
According to Beyondsoft's website, its content monitoring service, called Rainbow Shield, has compiled over 100,000 basic sensitive words and over three million derivative words. Politically sensitive words make up one-third of the total, followed by words related to pornography, prostitution, gambling and knives.
Workers like Mr Li make US$350 to US$500 (S$478 to S$683) a month, about average pay in Chengdu. Each worker is expected to review 1,000 to 2,000 articles during a shift.
Articles uploaded to the news app must be approved or rejected within an hour. Unlike Foxconn workers, they don't work much overtime because longer hours could hurt accuracy, said Mr Yang.
It's easy to make mistakes. One article about Peng Liyuan, China's first lady, mistakenly used the photo of a famous singer rumoured to be linked to another leader. It was caught by someone else before it went out, Mr Yang said.
Mr Li said the worst mistakes were almost all related to senior leaders. He once missed a tiny photo of Mr Xi on a website not on the white list because he was tired. He still kicks himself for it.
When asked whether he had shared with family and friends what he learnt at work, such as the Tiananmen crackdown, Mr Li vehemently said no.
"This information is not for people outside to know," he said. "Once many people know about it, it could generate rumours."
But the crackdown was history. It wasn't a rumour. How would he reconcile that?
"For certain things," he said, "one just has to obey the rules."