1. because Kate Moss does, and even Johnny Depp does (and everybody loves him), and they're still alive;
2. because you smoke if your friends do;
3. because you think you're immortal when you're 14;
4. because it's incredibly interesting to adopt something that no one is allowed to do any more, not even in a pub;
5. because, as a teenager, you have the kind of determination you need to force yourself to like the taste of old, wet, rotting twigs.
(How did this ever catch on, this setting fire to tobacco? Maybe we just like fiddling with things – paperclips, beer mats, hair. Maybe lighting cigarettes gave us something to do with our hands.)
So I know all the reasons why. But when my 17-year-old has a friend who gets through a couple of fags before she even gets to school, I think, what's going on here? How can she afford it? And, more importantly, how did she manage to ditch all that health information, all those ads on TV?
We're not living in the 1940s, when film stars eyed each other up through clouds of smoke and everyone though a rasping cough was incredibly glamorous. Nowadays we know all about lung cancer.
"Don't start," I say to my teenagers. "Just don't start." (Because I love them. Because I don't want them to die.)
And then I think, oh, be careful. Don't go on about it. There's nothing like your mum saying 'don't' to make something appear incredibly interesting.
So I cross my fingers instead.