Years ago, when I was a teenager, arrangements stuck. You met at 7pm outside the cinema, or at 11am in Top Shop. Your parents knew where you were going because there was only one phone in the house and they had heard every word.
It doesn't work like that any more. Arrangements are fluid. Facebook and mobiles mean that large groups of teenagers change direction suddenly, like shoals of fish.
'Where are you meeting?' I ask my 15-year-old.
'They'll text me when they're on the bus,' she says.
Theoretically, of course, I can keep up with this casual indecision by spending the whole evening composing my own increasingly frantic texts. 'Where r u?'/'R u ok?'/'Pls ring'. But, as we all know, mobile phones only work when they're a) turned on b) picking up a signal and c) in credit.
Even if, by some miracle, all these options are satisfied, the plaintive bleeping of a maternal text usually goes unheard in the thundering uproar of a teenage gathering. 'Sorry,' says my 17-year-old when he finally rings. 'It was in my pocket.'
Mobiles make me cross. They give the illusion of safety but don't deliver. I feel like Daniel Craig in Casino Royale struggling with a broken defibrillator.
'But we'd ring if there was a problem,' says my daughter.
Yes, you're right. You would, I know you would.
But I'd just love to spend a peaceful evening on the sofa imagining I know exactly where you are.
Catch up on Marianne's weekly columns on living with teenagers here.