Masturbation and the location of the clitoris are explained in a frank new cartoon booklet about sex for primary pupils.
A Christian group says it is appalled by the book's explicit drawings - including a cartoon of a girl apparently pleasuring herself in the bath - and its treatment of homosexuality.
But the Family Planning Association hopes the booklet 4You, published today, will answer many of the embarrassing questions that nine to 11-year-olds pose about sex.
Author Liz Swinden, a former teacher and veteran writer on sex education, expects controversy.
"It's less twee than some previous books and more in touch with what kids'
lives are like," she said "People complain about spoiling children's innocence, but they need to know about their bodies and how they will develop.
"Masturbation is an area where we really have taken the bull by the horns.
I told an American teacher about it and she said 'My god! You can use the "m" word?'"
Mrs Swinden said she felt it important to include a diagram explaining where the clitoris was. "I'm 56 and I didn't find my clitoris until my early 30s," she said.
On masturbation, the booklet says: "Masturbation isn't harmful, and it can be a way to find out about sexual feelings and learn about your body." And on homosexuality, the advice is short and non-judgmental: "Sometimes you fancy people you know. Sometimes you fancy someone famous. They may be the opposite sex from you, or the same sex."
The Christian Institute, a charity which promotes Christian values, said parents would be outraged. Simon Calvert, institute spokesman, said: "The book's highly graphic approach is wholly inappropriate for children of primary-school age.
"If someone stood outside the school gates handing out comic books on sex to nine-year-olds they would be arrested - but somehow if it's educational then that is fine."
Also unimpressed was Sue Clemson, a headteacher and former personal social and health education adviser, who said she would not be using it with her pupils at Bannerman Road primary in Bristol.
"Parts of the text are very good but I really didn't like the pictures - and I would consider myself open-minded," she said.