Your mum. Whether the words are whispered or shouted, they are enough to start a fight. It's that dreadful Year 8 class again. You are the teacher at the front trying to focus the group on the essential learning objective you've written on the board. The lesson is about using descriptive words to improve writing. You're going to guide pupils through a piece of writing in which their linguistic flair can be demonstrated. But you just can't cut through the "Your mum" factor in class.
Some pupils insist on showing their flair for language and vocabulary, delving into a rich imaginative vein of tone and colour as they insult each other and prevent the lesson starting. Indeed, the words "Your mum" give creative potential to an alternative curriculum that a teacher will find hard to control.
Mother cussing often occurs among pupils who are good friends, and engaged in as no more than a bit of light banter. There are, for example, the delayed teasers such as "Your mum... (long pause) smells very nice," or "I've seen your mum strip ... the bed." The rules of engagement state that this kind of exchange can be stopped when one of the parties starts to express annoyance.
But mother-cussing can become very vicious, with a tendency to become sexually explicit as pupils get older. I once noted a progression of cussing from Year 3 to Year 9, where the cussers in question used the local supermarket as a source of bizarre mother-cussing images. In Year 3, the cusser claimed: "Your mum is so dumb that when she got locked in Sainsbury's, she died of starvation." By Year 9, it had become the more vicious. "Your mum is so ugly that I had her with a Sainsbury's bag over her head."
Of course, mother cussing is only one strand of the repertoire of trading insults. Other forms often revolve around economic status: "Look at your clothes - they're from Oxfam" or, "Your family sleep in a shoe box." This is standard fare. But the richest seam combines body size with mother cussing - the "fat mums" or "ugly stupid mums" syndrome.
So why do pupils ruin the carefully prepared lessons with this? The main reason, they say, is for entertainment value. Yes, pupils are that bored in lessons! The phenomenon is mostly boy-on-boy, although other combinations are not unheard of. According to boys, dads don't get cussed because manhood must keep its good name. What the boys don't say is that the relationship with "mum" is often the most precious for them, especially in a one-parent family, so mocking her honour is likely to hit home. But machismo plays a prominent role in boys' mum-cussing - it makes them feel tougher, they often say. In this culture, being able to deliver a good mum cuss is important but it is even more character-forming to take a sharp ribbing and shrug it off. "It makes you harder, makes you grow up," said one lad. "It shows you where you are in the pecking order."
Girls don't really go in for "Your mum" jokes. In general, they are less prone to the florid conventions of cussing and tend to go straight for the jugular. So a direct, "You slag" or "You bitch" is more likely to be heard.
When putting a boy in his place, they are more inclined to pick on economic factors or physical appearance: "You look like a tramp in those clothes," or, "Your face looks like tomato pizza." Controversially, most of the teenage boys I interviewed saw this kind of comment as a sign that the girl just fancied them and was looking to get herself noticed.
So what exactly are the origins of the "Your mum" cuss and how widespread is it in our schools? I spoke to schools all over the country and asked them if they recognised the phenomenon. Their responses show that from small market towns, to picturesque country areas and in the big urban centres outside London - in every corner of the UK - the "Your mum" cult is flourishing.
It seems to have spread out of the streetwise African-Caribbean cultures of London and the big urban centres. Indeed, name-calling was already alive and kicking in these areas in the mid-1980s. But in the past five years or so, musical genres stemming from the United States such as "hip-hop" and "gangsta" have brought with them a worldwide movement of popular misogyny - witness, for instance, the lyrics of the likes of Eminem. This youth culture has globalised the culture of "cussing" and "dissing". An assistant head in the Peak District told me: "We get a lot of index finger in your face stuff now when disaffected pupils argue back with teachers like a Master MC."
Interestingly, that school had no pupils from ethnic minorities. It has always had a white working-class population.
Paul Blum is the author of "Surviving and Succeeding in Difficult Classrooms" (Routledge Falmer pound;11.99)
Five favourite cusses
* Your mum is so fat that her shadow weighs 1,000lb
* Your mum is so fat that when I punched her, pork chops fell out of her mouth
* Your mum is so fat that when she sits in the back of a bus it does a wheelie
* Your mum is so ugly that when she puts her head out of a window she gets arrested for mooning
* Your mum is so slow that she got hit by a parked car
How to cope with insulting pupils
* Become familiar with the culture in your school. Get to know the difference between a "bit of banter" and "spoiling for a fight"
* Keep it out of the classroom
* Prepare a few put-downs: "How would you like it if somebody said that about your mum?" Or: "I tell you what, I'll write down exactly what you said about his mum and quote it in a letter to yours!"
* Find out how colleagues deal with cussing. There might be whole-school approach