Coming back from a meeting, I stopped outside Euston Station to get myself an espresso for the journey. For non-Londoners, Euston is one of the few major stations in London where you can sit outside.

Going towards the cafe, I noticed a plywood board resting on a luggage trolley. A large comment, written in pen at the top: I'm smoking for God. The rest of the board had things stuck to it, collages, even a cigarette melted in wax attached to it. I was intrigued.

I wanted to see what was on the other side. Poking my head around, I noticed the owner; a not-unattractive girl who said:

"What ya doing?"

"Just wanted a look"

"Only if you give me a cigarette". I don't smoke, and said as much. "Bye, then", I said.

Her desire to show off her work was too strong, though. Thought it would be. "Come on", she said, indicating the place on the bench next to her. I sat down.

"Are you happy?" she asked me. An odd opening question. I had to think for a second, but I answered "Yes". My tone of voice was distant, and she noticed.

She took me through the piece, explaining each symbol. This is for Trainspotting, this in memory of a friend, this insulting a teacher. She asked my name about 20 minutes before I asked hers.

She was literally penniless; waiting for a lift, without enough cash to buy food or make a phonecall. I bought her a coffee; she bummed a fag off a passer-by. We carried on talking. She was seventeen, about to enter her final year of school. She had an opinion on everything. Robbie Williams, international politics, drug use, classism. And they were all so strongly held.

She seemed fascinated by how laid back I was, which is odd; I've always thought as myself as anything but laid back. She'd excitedly ask me a question. "I suppose", I'd answer, or "sort of".

From time to time, a parent would phone her, and she'd argue loudly, oblivious to me while I withdrew Foucault's Pendulum from my bag and read. Then, the call over, all her attention would be back on me again. She showed me her writing. and the prose wasn't bad; terribly spelled, but beautiful ideas. Some of it would fit in fine here.

"You shouldn't be so angry, so worked up", I said.

"That's easy to say, I bet you never get worked up."

Actually I used to get pretty worked up myself, but then I..."

"Grew up?" she completed the sentence for me, but it wasn't what I was going to say.

"I stopped." She seemed genuinely impressed.

"You stopped.", she echoed back, with a look of wonder.

Her friend arrived, and her plans to drag me to a pub evaporated. I took my leave, after she aked for my phone number.

Standing on the Tube, I got to thinking again. Looking at the tired, weary faces of the people around me, I began to remember that I am actually happy. The Tube is dirty, tainted, and sometimes it seems that everyone on it has caught the taint, the grime, the Underground depression. It shows in the eyes. But looking closer, the odd person shines out among the grime and I realised that it's not the London Underground depressing people but the other way round.

For a few weeks now, I'd forgotten that I was happy, and this chance one-hour meeting with a stranger reminded me about inner peace, which goes away if you forget you have it. I went home smiling, humming, and I still am.

At 11pm, she called me and asked to stay the night. I made some excuses. I suppose I'm a nicer guy than I realised.