'What,' she asked, 'is the average age that girls reach puberty?'
'12?' ventured the bravest of us. '14?'
'Eight!' said the lady triumphantly and wrote 'eight' on the white board.
There was a bewildered murmur from the audience. Only I was unsurprised. Puberty, I've learned, is not heralded by packs of Bodyform Ultra or incipient bristles. It begins with a rash of equally unnerving symptoms, none of which biology guides warn you about. And in our vicarage it commenced around the age of seven.
It's taken me a couple of years to realise what these symptoms signified and I was about to share my wisdom with the anxious mothers in the sex session. But before I could find my voice, the lady from the council had started sketching aspects of puberty that I do not yet feel mature enough to cope with and, with shrill excuses, I fled.
Here, though, for the benefit of those with small girls, I intend to reveal all. If you realise that your own infant daughter is showing any of these signs, then don't panic. It's a natural biological process - it's just that it starts so darn early in modern Britain.
You know your little one has reached puberty when:
You find metallic blue nail varnish smears on the bath tub.
Leopard print creeps into the house, like said nail varnish, by unidentifiable processes.
She becomes physically incapable of walking proper distances unless it's down a shopping mall.
She starts borrowing your shoes.
You start borrowing her jewellery.
She asks when she may start shaving her legs.
£1.50 pocket money becomes a £2.50 'allowance'.
'Will you do my bath, Mummy!' becomes 'Wot you hanging round the bathroom for, Mum!'
Her bedroom chaos morphs from Barbies, shoe-box castles and rolled-rug 'ponies' to Matalan catalogues, lip salves, clothes purloined from your wardrobe and mysterious twirly gadgets to perform mysterious twirly functions on hair.
You need a torch and ear mufflers to browse T-shirts in her preferred mood-lit high street supplier.
I'm bound to have left some crucial symptoms out. If you can think of any please add them here. And don't be disheartened. Puberty is a painful process for a parent, but there are plus sides: my 10 year-old still almost believes in Father Christmas