The 18-year-old is going InterRailing round Europe. She leaves tomorrow. She is singing as she packs her rucksack.
"You know," says her brother casually, "I thought to myself this morning, I must make sure I say goodbye properly because it'll probably be the last time I ever see her."
I look at him in horror. "Is that supposed to make me feel better?"
He shrugs. "I'm just saying."
All day I try to comfort myself with the fact that teenagers do this all the time – go off travelling, camp at summer festivals, sleep on the beach in Cornwall to celebrate the end of exams. It's very rare that anything goes wrong.
(Although it does. It does go wrong sometimes. Two years ago, a friend's son died in a coach crash in Thailand.)
Life, I tell myself firmly as I peg out the washing, is full of risks. Just crossing a London street is dangerous.
"How about," texts the mother of one of her friends, "we set up some kind of rota to let each other know they're all OK?"
I sit outside with the Sunday papers and watch the cat having a furious fight with a twig. Three weeks will pass quite quickly, I think. Before we know it, she'll be home again – happy, grubby and broke.
"She'll be fine," says my husband. "They'll all look after each other."
I open You magazine. There's an article about a 19-year-old student who was gang-raped in India.
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