In London, where we live, you don't have to pay for bus travel if you're in full-time education. (Or maybe it's if you're under 18. I'm not sure. But basically anyone who's at school doesn't have to pay.)
My teenagers used the half-term break to renew their cards. This involved filling in forms, getting passport photos, and queuing up at the post office.
'Do I have to?' said my daughter, who was already exhausted from getting up and eating toast.
But they were both quite cheerful when they returned. My 16-year-old clutched her receipt.
'If we show this to the bus driver,' she said, 'he'll let us go free while we wait for the cards to arrive.'
Or not, as it turned out.
A lot of people in positions of power don't like teenagers. I was shopping for a school shirt for my daughter in a well-known department store a few weeks ago. As we went to pay, we realised that she needed two shirts, not one, and I made the mistake of leaving her to look through a rack of cardigans while I trawled back through the store. As she stood there, alone, she was approached by a store detective who asked her what she was doing.
'Waiting for my mum,' said my daughter.
'What are you holding?' said the store detective.
'A cardigan,' said my daughter, because she was.
When I got back, I was so infuriated by the implication that my 16-year-old was a potential shoplifter that I wanted to make a complaint. But my daughter was mortified by the very idea.
'Don't,' she said, with panic in her eyes. 'That would be so embarrassing.'
So maybe I'm already bristling with fury. But yesterday, on a family outing to the cinema (it's my birthday, so I can demand that my teenagers come too), we're up against it again.
We get on the bus. My daughter says, 'My travel card's in the post.'
The bus driver says, 'Where's your receipt?'
'Here,' says my daughter confidently, showing it to him.
The bus driver scowls. 'I can't let you on without a card.'
I want to say, so why did you ask for the receipt? Just to prove she was lying? But it's my birthday, so I don't want to stand there arguing.
But I think, as I sit down, teenagers are guilty until proved innocent.
No wonder so many of them get cross.