Sometimes this is a deliberate tactic. "Can I have a few people over on New Year's Eve?", for example, when you're sponging white gloss paint off the kitten. (Don't ask.)
The hope is, obviously, that you're so distracted by the task in hand that you'll say 'Yes' without thinking.
(I like seeing my teenagers' friends. But I'm always worried about parties. You hear so many horror stories. One friend had to hire an industrial carpet-cleaner because someone thought it would be funny to sprinkle sugar on the kitchen floor so that sticky mud syrup was trampled throughout the entire house.)
But sometimes it isn't deliberate, this starting up of serious conversations when you're focusing on something else. It's just that 100% of your attention might be too much. Parents ask a lot of questions. Who? What? When? Where? And teenagers don't always have the answers.
So teenagers wait until you can't really concentrate. It's a safety measure. It's a way of testing the water without jumping in. If you're only half-listening, they can backtrack. If necessary – if they see your eyes widening in shock as warning bells start to clang – they can deny it all completely.
"Did you say you got a D?"
"No, Mum. You weren't listening."
So they come out with the big stuff when you're driving in heavy rain. Or when the onions are burning. Or when there's something on telly you want to watch that starts in five minutes.
They talk about stomach pumps and drugs and depression and teenage pregnancies as you're negotiating a right-hand turn in front of fast-moving traffic.
"So who got expelled?" you say, going round the roundabout a second time.
"I didn't say anyone got expelled."
On New Year's Eve, one of my friends had her head halfway through the hatch to the loft (looking for the inflatable mattress, I think), when her teenage son, holding the ladder a few feet below, told her he was gay. She wobbled back down.
She said, "But why did you pick that moment to tell me?"
He said, "I didn't want to start the New Year on a lie."
What she meant, of course, was why wait until I'm half inside the attic?
But that's what teenagers do. From his point of view, the timing was perfect.