President Obama, Russian President Vladimir Putin and Chinese President Xi Jinping at the Asia Pacific Economic Cooperation (APEC) Summit in Beijing in November. REUTERS/Alexei Druzhinin/RIA Novosti/Kremlin

BEIJING – Chinese censors have never been known for their sense of humor, but their lack of it was amply demonstrated this week when they blocked a child’s light-hearted suggestion that President Xi Jinping might like to lose some weight.

As part of a school writing project, nine-year-old Niu Ziru wrote a letter to Xi suggested he plan a mission to Mars, but also noting he might like to lose a few pounds.

"Xi Dada, you could lose some weight,” he wrote in the letter, using a nickname for the president roughly meaning Uncle Xi. “You don't have to look as skinny as Obama. Like Putin is okay! J"

The letter was never sent but Niu’s father, who thought it was amusing, posted it on social media. It went viral and was picked up by mainstream media, only to be pulled from major domestic news Web sites on Wednesday by censors, while direct references to the boy or the letter have been scrubbed from social media.

Niu, a Grade Four student at the Best International School in the city of Zhengzhou in Henan province, made the comments as part of a letter about the space industry. He reportedly told the Zhengzhou Evening News that he had seen Xi and other leaders during the recent APEC Asian leaders summit, and thought Xi had that "presidential look" and had "made Chinese people proud, except that he’s a bit fat.”

In October, Hua Qianfeng, a nationalistic blogger who has been praised by Xi, said in an interview that the Chinese leader was “not fat at all, he is very strong.”

Xi’s down-to-earth image might have encouraged the boy’s friendly tone, but the incident made it clear that the officials who run Chinese propaganda and censorship are as uptight as ever.

-- A full translation of the letter, in which Niu addresses the president as “Grandpa Xi Jinping” follows:

Dear Grandpa Xi Jinping,

Greetings! I’m a pupil from Zhengzhou, Henan Province. Today I want to talk to you about space. Everyone believes the next big challenge is space. I think so too. The moon, for sure has rich minerals, but it doesn’t have air and water and is definitely not suitable for human life. This is not a good place for a long stay. It’s time for China’s aerospace program to change directions.

Let’s have a look at Mars. It has both air and glaciers, it's just as good a place as Earth. Eventually, the United States and Russia will no longer use the international space station and the US and Russia are planning full force to land on Mars. Even the European Union and India are joining the fun. Let’s also hurry up!

Okay, let's move on to some lighter subjects.  Xi Dada, you could lose some weight. You don’t have to look as skinny as Obama. Like Putin is okay!

Best regards,

Wishing you good health

Niu Ziru

Xu Yangjingjing and Xu Jing contributed to this report.