So how do crabs do it?
There is no other topic so guaranteed to bring out a chorus of sniggers, snorts and muttered innuendo. Maintaining respect and dignity while discussing what goes where and how can challenge even the most self-assured teachers.
But, in an attempt to help those embarking upon the ordeal, contributors to The TES online staffroom have submitted samples of queries likely to arise during sex-education classes.
Often, back-to-basics clarity is called for. One contributor describes how, when dealing with a class of 10-year-olds, she was faced with the question:
"I know you have to have sex to have a baby. But if you want to have another baby, do you have to have sex again?"
Other pupils are keen for a more sophisticated understanding of the process. One teacher was asked why condoms are flavoured. Another had to contend with the query: "Do you have to make noises during sex, or is it just tradition?"
This contributor continues: "It is advisable to get pupils to hand questions in at the end of one lesson, so you can vet them before the next lesson."
Turning to animal analogies could present its own problems. One teacher was forced to explain the mechanics of elephant sex. Another was asked: "How do crabs have sex without pinching each other?"
John Trip, originator of the A Pause sex-education programme, said:
"Teachers should answer questions practically and clearly. But they should also think about the purpose of the lessons and whether they need to answer every question."
It was perhaps inevitable that one teacher would receive this response from a Year 7 class: "Are we going to do practicals, miss?"