Do all small children have a mild form of Tourette's Syndrome that makes them shout inappropriate things in public, or is it just mine? I realise that kids are notoriously unpredictable creatures and their grasp of social niceties is generally pretty precarious, although this could have something to do with their gooey little hands, but what kind of primal instinct is it that prompts them to shout 'bum' on the bus?

You could argue that it is nurture rather than nature and that loud, inappropriate children have loud, inappropriate parents. But that wouldn't explain why even those who wouldn't normally say boo, never mind bum, to a goose can't seem to control themselves when faced with a room (train, museum, church - delete as applicable) full of strangers.

Take my own munchkin, for example. He's normally a quiet(ish), sensitive little thing (just like his mum) who minds his Ps and Qs and keeps himself to himself. But put him anywhere where he has a captive audience of strangers who are not paying him any attention, and you are virtually guaranteed an inappropriate eruption.

He has two stock phrases that he unleashes when he can't think of anything more embarrassing. The first, a hearty 'cor, blimey!', he saves for occasions when the mood is distinctly quiet and sombre, and the second, a wide-eyed 'excuse you, grandma' (or whoever else he happens to be with), he uses whenever he finds himself in the company of easily-shocked old ladies.

These are of course pretty tame exclamations guaranteed to raise an eyebrow or a laugh rather than the blood pressure. Sometimes, however, he really outdoes himself.

Take for instance the time we emerged from the loo into a crowded restaurant and he yelled gleefully 'Mummy's pooped!'. He knew that I had done no such thing - he's still at that stage when his parents daily ablutions are only marginally less fascinating than a double bill of Bob the Builder - but he also knew that this was a situation when only a corking one-liner would suffice.

And it is not only me that the little cherub tries to embarrass: he doesn't spare his father's blushes either. Imagine the chaos he caused on Christmas Day when, in a lull in festivities around the dinner table, he informed his great-grandma that 'Daddy's got a big clanger'.

It was only after his red-faced, and yet somehow proud, father explained that the clanger in question was a stuffed toy given to him by his brother, that great-grandma Madge felt able to continue tucking into her turkey leg with her usual gusto.

Sometimes the munchkin gets bored with torturing his parents in public - we're just too easy - and decides to spread his net a little wider.

Take the time we were enjoying a drink and a sticky bun in the Spurriergate Centre one Saturday afternoon and he casually leaned over to a teenager at an adjoining table and said 'you're not such an ugly duckling'.

Or, and it still makes me shudder to think of this, when he attended his cousin's christening in a packed Peckham church and vigorously shook his head and shouted 'no!' when the vicar asked if we all denounced evil. Perhaps we should have named him Damian instead of Jack.

Even after all my first-hand experience, I still have no idea why children (it's not really just mine, is it?) yell 'knickers' in the supermarket, but I can guess. You only have to look at the glee on their faces to see how liberating it is to let rip with whatever comes to mind.

Maybe we adults should turn the tables occasionally and have an inappropriate explosion of our own while waiting at a bus stop or queuing in the post office. Just imagine how liberating it would be, and how much it would embarrass our children.

Come on everybody: 1...2...3... bum!