Great build-up, pity about the punch-line

'WHAT?' I say, already nervous, already suffering the first pangs of stage fright. 'A joke? A joke?'

My first reaction is: no, not at all, I can't think of any jokes, apart from the one about the man waking up next to his wife, which just would not work here . . . but, I've left it too late, I've dithered for two or three seconds, so I'll have to do it, have to tell one. I smile, bemused, playing for time, racking my brains, finding nothing.

For the sake of form, I say: 'I don't know any jokes,' which comes out badly, not like a sincere statement at all, and I look at the people sitting at the table with me, assessing them, having at most 20 seconds to make up my mind, and for a terrible, suicidal moment I think: do the one about the man and his wife] Go on] I turn away from this idea sharply, finding not relief but panic.

There's an older man here, I've never met him before tonight, and a prim-ish couple, probably mid-thirties, who have dragged the mood of this dinner party in a slightly staid direction, which might, of course, work to my advantage, but . . . no, I'll have to think of something else. I haven't lost every shred of hope. I clear my throat and look around; effectively, I'm fully committed now. I've practically signed documents.

Why don't I always keep a joke handy? I can't think of a single reason. One flashes through my mind, the last joke I heard, a nasty little thing that starts: 'What's 20ft long and smells of urine?', and I dismiss that instantly. I clear my throat again and take a sip of wine, and I realise that, right now, the audience is still rooting for me, I'll still get the benefit of any doubts, but this moment has very nearly gone.

What was the last genuinely funny thing I heard? Someone told me yesterday that he was talking to one of his colleagues and the colleague said: 'Well, it's a doggie-dog world,' and he said: 'Don't you mean a dog-eat-dog world?' The guy says: 'What do you mean, dog eat dog?' So he says: 'Well, dog eat dog, you know.' And the guy says: 'I'd never heard that.' He'd spent his whole life thinking that people said: 'It's a doggie-dog world.' I laughed and laughed.

But here, it's useless. These people need a joke. A joke is a formal thing, with a proper structure, a kind of syllogism that short-circuits language, gets right to the heart of something else - your real feelings, perhaps. Yes] I've thought of one] How many hairdressers does it take to change a light bulb? Two - one to change the light bulb, and one to stand back and say: 'Fabulous, Gary]'

Right now, looking around this table, at the mess of food and plates, the drink-puffed faces, the hairdresser joke seems - what? Too short? Too unfunny? If I'd just snapped it out, straightaway, maybe I could have got away with it. But now . . . too late. 'So . . . what sort of joke would you like?' I'm smiling, desperately trying to remember something I thought was funny a couple of days ago, about a Chinese man masturbating in a cupboard. What was it? The details seem hazy, half-formed. A block of flats was involved, a thunderstorm, Arnold Schwarzenegger. But I haven't quite got enough grip on it. Sometimes, of course, you can just launch into a joke, take the risk, and it will all fall into place. But if you put a foot wrong . . . Somebody once told me: 'There was this guy in Moscow, dying to go to the lavatory, and he asked a local where he could go, and the guy says, go to the American Embassy and do it on the floor, so he goes to the American Embassy, and does it on the floor.'

'Yes?'

'Well, hang on. No, that's not right, is it? He says, yes, where can I go, and the guy in the street says: 'Go into that building, they let you do it on the floor.' So he does, right.'

'Right.'

'Oh, and . . . yes, then he comes out of the building, and it's only then that he sees the sign: American Embassy.'

So I'll have to be really careful; in a joke, the balance between success and failure is extremely delicate. A punch line is hovering into view: 'No thanks, I'm fine - I've been eating these gherkins all night.' But no - that's . . . disgusting. Like almost all jokes, it wraps the unsayable, the dodgy, in a translucent package, so you look at it for a moment and, then, all of a sudden, you see it. And you think: thank God that horrible, unsayable thing wasn't actually said. You laugh from relief.

I clear my throat a final time. I'm down to the man and his wife. It's about a man who gets so drunk that he completely blacks out, doesn't know what he's doing, and ends up in bed with a woman.

What? Is this a funny enough premise? The guy isn't in much of a moral dilemma; there isn't much narrative tension. But you have to look at your audience and think: what is their precise level of unsayability; how much translucent wrapping do they need? At least, with this joke, I won't have an obscenity crisis, the terrible thing that happens when you've leapt off the parapet of a joke and suddenly find yourself free-falling through hideous filth you'd forgotten was there - you're tumbling out of control, in a hot and cold sweat, blurting out the names of body parts, the punch line a tiny dot on the landscape.

My time is up; I plunge in. The first line gets a giggle. I'm in] The rest I string out, milking every development.

' 'I woke up, hung over, saw there was a woman next to me . . . so I gave her pounds 20.' ' The older man is grinning at me, swinging his head. The prim wife is gurgling.

' 'And then I thought: oh no, she's my wife]' '

The laughter starts, a neurological reaction racking these people's bodies; all it takes is a story, the gist of which is: a man mistakes his wife for a prostitute.

'And the guy says, he says: 'It wouldn't have been so bad. But she gave me a tenner change]' '-