The carriage is relatively empty. An old lady studies a Sodoku pamphlet on the seat opposite. Two Japanese students are asleep with their feet up, to my left. We are all quiet and anonymous, which is the law.

But just several metres away stand three drunk men, roaring with laughter. The students sleep through it. The old lady concentrates on her Sodoku. The men roar with laughter again. They are talking about the Japanese students and pointing.

'Him, right,' says the first one, thinking he's whispering, 'he's probably, like, a karate instructor!'

His friends splutter and high-five each other.

'What about his mate?' bellows his friend. 'Guess his job!'

'Him? Probably, like ... a shark ... fisherman?'

His friends don't find this quite as funny, and instead of laughing, they shrug and sip at their beers. But they want to recapture the brilliance of their pal's first incredible observation, so they persevere.

'What about him?' says one.

They are pointing at a man I hadn't even noticed, slumped against the window, a copy of The Times on his lap and a little bit of drool inching its way from his mouth.

'Him?' says the first man, now with everything to prove.

'Yeah!' say his mates. They can't bloody wait for this.

The first man looks at the sleeping stranger. He has a window of approximately two seconds in which to make his joke, otherwise it will wilt and die.

'That bloke probably works ... at a drool factory!'

It is rubbish, but it is quick, and sometimes that's all that matters. His mates go crazy, whooping and stamping the floor, but still thinking they're being very quiet indeed.

The old lady closes her eyes. We both know I am next. I close my eyes and pretend to be asleep.

'What about that one?' says one of them.

'That one with the glasses?' whispers the first.

There's a pause as we go through a tunnel. The train screeches and the carriage jolts and the lights flicker slightly. It gives the first man valuable thinking time. I hate this. I hate it when people look at you on the tube. It's not right. It shouldn't be allowed. The whole point of sitting on the tube is not to draw attention to yourself. We exit the tunnel.

'I reckon that bloke with the glasses is either ... a librarian ...'

His mates start to giggle.

'... or ... a male escort!'

They erupt. It's a home run.

'A male escort!' says one of them. 'Brilliant!'

'A librarian!' says the other, but I'm not convinced he's on the same level as his mates.