Every so often, one of the children will come up to me and say: "OK, now: shut your eyes and open your mouth." Or sometimes they say: "OK, now: shut your eyes and hold your hand out." Yes, a surprise is imminent. The first one is better, because it will probably be nothing more than an unexpected, and possibly slightly squashed, blackberry or raspberry. If it's the hand, it might be something not considered suitable for the mouth. A slimy creature they have found outside, perhaps, or even something amusing that they learnt at school, such as a Chinese burn or that even more painful trick which involves jabbing a fingernail against the cuticle of one of your fingernails. It feels surprisingly like having a needle jammed into your finger but, happily, the pain is brief.

But a basic level of trust is assumed. There was a Press Association report last week of a 14-year-old boy (who cannot be named for legal reasons) from Ripon in North Yorkshire, who was having a piano lesson. He told his teacher that he had a present for her, but she had to close her eyes. Then he stabbed her in the stomach with a carving knife that he had taken from her kitchen. She tried to escape, but he chased her and she received further wounds in the face and body.

You may imagine that the boy was suffering from paranoid schizophrenia or some such psychological disorder, but this wasn't so. When he was arrested, he told police that he had intended only to cut her arm "to persuade his parents to let him give up one of his instrument lessons to play football". He was sentenced to six years in a youth detention centre. Whether music tuition features in the curriculum was not revealed.

I tried to reconstruct the boy's logic in my mind. You'd think it would be better to pretend to be ill, but he probably reasoned that his parents would say, as parents always do, "Well, if you're too ill to go to your piano lesson, you're too ill to play football". He could have stabbed his own hand, but that would have met the same response, and it would have hurt as well. That seems clear. What is harder to understand is the next step, in which he said to himself: "Hmm, those won't work, so I'll move on to plan C, a knife attack on Ms Thomson-Jones."

I have attempted to come up with a serviceable moral from this story, but all I can offer is a lesson that I've learnt from years with my own untrustworthy children. When anybody tells you to close your eyes, pretend to close them, but keep them open by about half a millimetre. It's not exactly epigrammatic, but it might be worth adding to the three other pieces of classic advice: never eat at a place called "Mom's"; never play cards with a man called "Doc"; and don't sleep with anybody whose troubles are worse than your own.

I could also have mentioned the notion that music has charms to soothe the savage breast, but I don't think anybody would make the same claim for music lessons. I've read that when animals, especially pigs, are led into a slaughterhouse, they somehow sense what is going to happen to them and they start to squeal in a terrifying manner and struggle desperately to escape. My youngest daughter behaves in precisely the same way when she starts to suspect that violin practice is in the offing. And yet, sociological research shows that the main regret of 90 per cent of adults is that they can't play a musical instrument.

In any case, I have long shed all my illusions about the arts as an ennobling force. Witness, to take a current example chosen virtually at random, Salman Rushdie, who in a recent interview for the New York Times said that among his reasons for moving to New York City was that London's literary world was "bitchy, backbiting and incestuous". A report in the Observer quoted an anonymous "literary commentator" as being sceptical about this, saying that the real reason was that his recent novel, The Ground Beneath Her Feet, "did not sell well and a lot of people were saying Rushdie had got a bit too big in the ego and would not allow any changes to be made to his draft". Nothing to do with bitchiness or backbiting at all. Got that?

Oh, by the way, readers may recall my column last week, in which I discussed the curiously inappropriate phrases used in Guardian obituaries. Here is a quotation from this week's obituary of Paula Yates: "But it is her indomitable spirit that people remember and her wholeheartedness in life and love. Paula Yates was a survivor." Sadly not.