“Every time you flush the loo, droplets of contaminated water are released into the air. These can travel up to several metres.”

That is how my sister-in-law’s Nissan Micra got bloodstained in the municipal car park, despite her being some distance away when the mugger punched her in the face and stole her handbag.

“I would have handed it over,” she explained. Some feat when you are suffering from a broken nose and severe dental trauma.

The policeman comforted her. “Don’t take it personally, Madam. We know him well. He punches all the women he robs.”

“Does the aerosol effect apply to butter cream?” I asked, from the safety of the bar area. I thought it might clear up the mystery of my daughter’s third birthday party. In 1987, Tristan, the son of a psychologist friend, smashed up my daughter’s cake. She was only three and can’t remember it, but it stuck in my mind.

It also stuck to his fist, the tablecloth, and the wife’s glasses. Shortly afterwards, some discoloration appeared on the kitchen ceiling, which several coats of emulsion failed to eradicate.

Tristan’s mother believed children should never be punished. She maintained this position through several kicks to the shin, a head butt to the groin, and a Tonka Truck to the solar plexus.

Unfortunately, after I explained my view - badly behaved children should be thrashed to within an inch of their lives and sent to the Antipodes without supper - our friendship became strained.

So strained, in fact, that I didn’t see her again until she emerged, several years later, as part of an advisory team delivering training on How to Manage Challenging Behaviour in the Classroom.

There were lots of case studies, showing how teachers failed to spot the early warning signs and didn’t displace aggressive behaviour.

But suppose child X enters the class wielding a big stick and is intent on smashing someone’s head in?

Have an action plan ready. Evacuate other children. Get behaviour support to initiate calming strategy. Lead X to mentor’s room. Complete calming process by allowing X to play on computer for 15 minutes. Re-admit X to class. It works, too, several times a week.

It took a fortnight for my sister-in-law’s face to resume its familiar shape and colour, during which time I advised her on a range of calming and de-escalation techniques for next time.

“Look,” she said, pausing in her vigorous attempt to polish the blood out of her Micra’s paintwork, “if you don’t shut up, I shall do to you what Tristan did to me.”

“Tristan?” I gasped. “They called the mugger Tristan?”

Steve Eddison, Key stage 2 teacher, Sheffield.