China censored the letter N from its internet for at least a day.
The ban came as China cracked down on online discussion over the Chinese Communist Party's proposal to scrap presidential term limits.
Abolishing term limits would allow President Xi Jinping to rule indefinitely.
It's not entirely clear why the government targeted N, but we have a few theories.
China temporarily banned the letter N from being published online after people started using it to criticize a plan which paves the way for Xi Jinping to rule the country indefinitely.
The Chinese Communist Party on Sunday proposed to abolish the two-term term limit for the president and vice-president, sparking an online backlash which it has been trying to control.
Critics flooded Weibo and WeChat — China's version of Twitter and WhatsApp — to protest the plan, but were swiftly met by the country's censors.
Various Chinese characters for terms like "emigrate," "lifelong," and "I disagree" were banned, alongside ... the letter N.
The screenshot underneath shows a user on Sunday attempting to type "N" into Weibo, and receiving an error message that the "content is illegal."
The other words that appeared to be banned in the screenshot are: "Xi JinP," "emigrate," and "indefinite control."
As of Tuesday morning local time (CST), China had lifted its ban on N again.
The letter is used in China to represent unknown numerical values, like the letter X in algebra.
Professor Victor Mair, a China expert at the University of Pennsylvania, said in a Monday blog post it was "probably out of fear on the part of the government that 'N' = 'n terms in office,' where possibly n > 2."
CA Yeung, a Perth-based China blogger, also posited that "N = infinity."
It could also represent "no" in Y/N select items. As Twitter user Kasumi Shen said: "You can't choose N in a (Y/N) select item as long as you are still living in China."
Images of Winnie the Pooh were also banned on Chinese social media. Xi critics often mock him by posting images like the one above which imply that he looks like the fictional, honey-loving bear.
Chinese state media, for its part, has been trying to play down China's latest crackdown on internet communication.
In a Tuesday editorial, state-run newspaper the Global Times accused Western critics of "hysteria" over the government's latest round of censorship.
"The biggest reason for all this is that the rise of China has reached a critical point where some Westerners cannot psychologically bear it any longer. They wish to see misfortune befall the country," the Global Times said.