Renowned mathematician Li Daqian admitted that writing the answers proved to be challenging sometimes.

"With limit on length, I was still asked to explain a certain issue thoroughly and inspire readers to think by themselves," Li said. He added that to ensure that the language is appealing to children and teenagers, he reviewed his first draft many times and revised all parts that may have been difficult to understand.

Despite the difficulty involved, the children's encyclopedia managed to invite the most qualified people to answer the questions. China's first astronaut in orbit, Yang Liwei, wrote a number of the pages on aerospace and space exploration. Scientists on manned deep-sea research submersible the Jiaolong, which dived to a depth of over 7,000 meters last year and has the greatest depth range of any manned research vehicle, became the authors of the entries on deep-sea exploration.

The newest edition has expanded its contents to include the latest scientific discoveries.

Wang Pinxian, an academician with the CAS and editor of the installment on the oceans, said that since the last edition came out in 1999, a lot of changes have occurred in the field of marine studies. While the ocean volume in the 1999 edition mainly covered maritime transportation and fishing, questions about deep-sea oil and gas exploration appear in the latest edition, which shows new research focuses in the field.

Keeping with the times

When the fifth edition of 100,000 Whys was released on September 18, 1999, the line of buyers outside the Shanghai Book Town extended for more than a kilometer.

"That was the peak of 100,000 Whys and the last prime era for pop-science books, when they still had the largest market share in children's books," Hong Xingfan, Deputy Editor in Chief of the Juvenile & Children's Publishing House, told Shanghai Daily.

The market share and popularity of pop-science books aimed at children dropped rapidly after 2000, yielding to children's literature and comics such as the Harry Potter series.

Among the 600,000 sets of the fifth edition sold, about half of those were sold in the first two years after its release. The publisher expects the sixth edition to circulate a total of 80,000 copies, including 50,000 hardcover sets and 30,000 paperback sets.

Compilers said that although all the questions in the latest edition can be found on the Internet, answers of the same quality cannot be found there, as scientists tend to provide the most thought-provoking answers and often explore more than one possible cause in their responses.

According to Hong, the sixth edition of 100,000 Whys has been seeking out resources from the Internet ever since the preparation stage, when questions were solicited on the Internet. Moreover, the official website and the 100,000 Whys account on Sina Weibo, a micro-blogging service, have gone online well in advance of its release. Netizens can raise their questions through the official website or leave a message at the Weibo account, which will be answered by experts hired by the publisher.

He said he expected that the official website, which was launched in April 2012, could be seen as an extension of the printed edition as some equally interesting and meaningful questions could be discussed there.

The Juvenile & Children's Publishing House is also cooperating with the CAS to produce a series of video clips to better illustrate some of the questions in 100,000 Whys. The publishing house also has a plan to design products such as a magazine, comics and video games.

"The production of a digital edition of 100,000 Whys has started and the publishing house plans to develop more than 100 varied products related to the series over the next few years," Hong said.

Email us at: lili@bjreview.com

Excerpts From 100,000 Whys

Why does the smile of the Mona Lisa seem mysterious?

The sense of mystery does not come from the woman in the portrait, but its viewers feel it when their gaze focuses on different parts of her face. When admiring a portrait, we often gaze at the model's eyes first while the mouth, the most expressive organ of a smile, falls into the viewers' peripheral vision. Due to the blurry quality of the peripheral vision, the curvature of Mona Lisa's smile face may seem larger than it is. When viewer's eyes then shift to her mouth to confirm the smile, the smile appears less obvious or even gone. Therefore, our brain's visual processing is what makes the Mona Lisa's smile mysterious.

Why do both our eyes always move together?

The six muscles that control movement of the eye are known as the extraocular muscles. They are connected to the brain by three cranial nerves. Although each eye has its own muscles, they are all controlled by the same signals from the brain that they share and that are carried by the cranial nerves.