SHANGHAI - The city's first sex education textbook for primary school students will be used in 18 schools in Shanghai in the coming semester.

The textbook, Boys and Girls, which the authors prefer to describe as gender education rather than sex education, was written by the Primary School Affiliated to the University of Shanghai for Science and Technology, and will be piloted in 18 primary schools in Yangpu district.

"The reason why we call it gender education rather than sex education is that the textbook contains not only biological, but also psychological and moral knowledge," Xu Jing, vice-president of the primary school, told media.

According to Xu, the textbook has been written based on the psychological characteristics of pupils. It uses metaphors and lively situations to explain sex and teach the pupils how to protect themselves from sexual assault.

For instance, in the chapter called "Where am I from?" the textbook compares the process of fertilization to a swimming race among sperm, who wear swimming goggles and practice very hard, with the winner having the honor of staying with the "ovum princess".

"Such language makes it easier for pupils to understand," Xu said.

Xu added that all the teachers involved have been receiving monthly training since last semester.

"I think the textbook is good for pupils," said Lu Hong, a white-collar worker in Shanghai, whose 6-year-old son will enter the primary school in September.

"Sex education is necessary for children. My son started to ask me where he came from two years ago," she said.

"But the language used for sex education should not be too direct," she added.

Lu said that there are a lot of sex education books for children in online stores, and she has already bought a collection for her son, which use a similar style to that of the textbooks.

However, Gu Jun, a sociology professor at Shanghai University, said that there is no need to use metaphors to explain sex to children and that sex education for children should start as early as possible.

"The earlier they are taught about sex, the less impact the knowledge will have on them, because children can easily accept all kinds of knowledge," said Gu.

He added the fact that parents use metaphors to explain sex to their children only shows their own inhibitions.