Triplets. Or, crowd mayhem.

I saw a TV programme ages ago about triplet girls. They were twenty-something and took delight in fooling a wide range of people, including a photographer, about who they really were.

One triplet would be Janine, and then a sister, Not-Janine, took her place. The photographer never knew she was a different person, because he never noticed, despite pointing a metre long zoom lens straight at her face.

Then Not-Janine took the place of The-Other-Not-Janine until it all became totally confusing for everyone involved, especially the shaking photographer, and the triplet girls thought it was all a fantastic laugh.

Until one of the triplets broke that magic accord. In the beat of a heart, she did or said something that crossed a silent, unknowable line; like the secret line you draw between a sibling and yourself and you know it is there and no-one else knows it is there. But it is. And before you can blink, everything falls silent because you know the boundary is crossed. The TV screen with those triplet sisters fell into stonysilence.

So much stonysilence in fact, that you could feel the cameraman stop breathing with the tension from that dead and leaden air while those mirror image triplet sisters stared unblinking at each other in a way which said, and meant: You are now going to die.

Those triplet sisters were aged in their middle twenties.

My triplet daughters are aged in their middle nines, and they tend to lack the inhibition that develops from the teenage years.

So when one sister strays across a forbidden sister line, let's hope they are not holding anything in their hands. If they are, you will think Tesco is being quite reasonable not to sell teaspoons to the under 25s.

Why I am posting this is for many reasons, but let's say it is because we spent today in London at the Royal Institution for their Family Fun Day.

With triplets, you just never know whether it will be fun or not. One moment here they are, all best friends, all together, having the same fun in total agreement and laughing in mutual accord. Then something happens. Like today.

I couldn't honestly tell you exactly what caused that triple combination of mayhem, death and annihilation. Perhaps it was ignited by a misplaced comment, a look, a turn of a body, a gesture, a glance, a frustration, a jealousy, a resentment. But that igniting spark transfers quicker than light and sound to the brain and body of a triplet sister and then you, foolish, inconsequential parent-type being, will be left flapping and opening and shutting your mouth like a goldfish trapped in a cardboard box.

When one face of my darling daughters stares, mute to the others, her face is talking and it says something like Now I am going to kill you. And, sure enough, that pattern is blasted all across those three mirror faces like fractured wallpaper, and probably on mine, too.

Someone who lives outside this strange triplet dynamic will very easily blame home education, because that is a good reason why everything goes wrong. So an onlooker from the crowd might see, tsk, and think, Aha! We all know why this is! Children lacking socialisation!

But if we are lucky, we can nip it in the bud, drag each sister off to a different room and pretend nothing triplet-type happened. If we are unlucky, within the time it takes you to clap your hands, we are dealing with a flaming explosion of protest. It is totally unstoppable and possibly accompanied by self immolation. Dignity is not on the agenda. Let's just say that any one of those flaming balls of exploding outrage will have a destiny made complete if every living atom is taken down to Hades.

So if you meet the triplet sisters, be prepared. It could swing either way. You will be charmed, or alarmed. Wear body armour, discreetly, just in case.

And we can always blame everything on the photographer.