An unwanted thought comes to mind: 'You know that story I was telling yesterday about Great Grandma's lavatory light switch and the chipolata?' I say.
'Oh yes!' he exclaims, brightening.
'Just make sure you don't use that one.'
There's a crestfallen pause. Then he perks up.
'What about the thing you and Auntie did at Christmas with the Brussels sprout?'
'That,' I say firmly, 'is not for publication.'
Further silence. A procession of memories discomforts me. I censor each one of them and cast urgently about for an example of wholesome hilarity which will show the vicarage in decorous light in the school staff room. 'I suppose you could use the humping game,' I suggest doubtfully. The humping game is a high-suspense competition involving ant hills and always takes place in public. The 12-year-old groans: 'That's not funny, Mum, that's just sad.'
I decide that the safest option is for the 10 year-old to make something up. 'But don't make it embarrassing or undignified,' I say, 'and show it to me first because Dad has to face school assembly tomorrow.'
A short while later he returns with a scribbled sheet. 'Mum and Auntie were washing up,' he reads out. 'Us kids went rushing in because of the terrible noise. They were both wearing saucepans on their heads and yelling a Tina Turner song into a potato masher. We thought it would be safer to join in so we all grabbed kitchen utensils as microphones and then Grandpa stuck his head round the door and waved his good leg to the music and Dad rushed in to complain about the noise, but even he couldn't resist and he grabbed a whisk and began boogying round Mum's tea towel.'
'That,' I say, 'is very original. There's only one problem.'
'What's that?' asks the 10-year-old.
'You might have to read it out in class and, as you know perfectly well, it's the gospel truth!'
Auntie in washing up mode