More specifically, writing sex, it being around Valentine's Day and all.

Sex scenes are the acid test of writing. They contain almost every problem that a scene can have, to wit:

- You have to describe physical activity. Physical activity is difficult to describe without descending into 'he did this, then she did that, then he did this, then she did that...' and getting flat.

- There's a lot of emotion going on, whether it's the first romantic love-making of the hero and heroine, a nasty sexual scuffle in the back seat of a car, or a bored middle-aged couple pretending to each other that they're not disappointed. Sex is always emotional - and emotionless sex is strange, and hence even more important to handle well. You have to get the tone right.

- In English, at least, the vocabulary for describing sensations is pretty limited. You've got 'pleasure', and 'arousal'; possibly 'tingle' and 'heat', and, well, that's about it, unless you're going to include words like 'itching' and 'discomfort', which may not be exactly what you're aiming for.

- Ditto for the sounds people make. Sex very often involves people making a variety of non-verbal noises, and while moan, groan, gasp and grunt describe some of them, they don't cover everything by a long way, and all of them have strong emotional overtones that may well be wrong for the scene. And worse still, the standard 'Aah' noise that most people make in response to a strong sensory stimulus, either pleasant or painful, is not accurately conveyed by any of them. Writing a sex scene in a language that denies you a word to describe 'going aah' is working under a dreadful disadvantage.

- It's watching characters do something that people don't ordinarily do in public. The intimacy ratchets up, and hence the reader is going to perceive things more intensely. Readers usually meet the characters like they'd meet a person socially - and suddenly hearing about a social acquaintance's perineum is, to say the least, mildly startling. The feeling of the conversation changes.

- There are a lot of cliches to avoid.

- It's easy to slip into generic sexual fantasy, meaning that your precisely detailed, finely-drawn characters are suddenly acting with uncharacteristic blandness.

- Many a good writer finds sex scenes force a sudden lurch in writing tone. The rest of the book was a thriller or a family saga, but these three pages are out of some pink-covered supermarket romance.

- The author's insecurities about beauty can kick in, leading to some odd inconsistencies. The heroine has been comfort eating for chapter after chapter, but take off her skirt and the hero finds himself kissing the 'flat plane of her stomach'. The hero's spent the last eight weeks performing non-stop miracles in the emergency room, which you'd think would cut into his gym schedule, but when the heroine reaches back, 'taut buttocks' await her grasp nonetheless. It's not just the implausibility that's a problem, it's the sense of anxiety that it implies. Western ideas of beauty are vicious, and when an author breaks off a sex scene to reassure us with irrelevent compliments about the characters' bodies, you can hear the echo of an appeasement chant to the Gods of Looking Good: 'It's all right, they can have sex, they're attractive enough for it to be legitimate.' But it doesn't work. It's a nervous aside that's worked its way into the scene. 'Burning with desire, John fumbled the catch of her skirt, unsnapping it at last to reveal her delectable hips (it's okay, though, they were only size twelve, so it's not as if there was any cellulite there to put him off) and buried his face in her stomach (not that there was very much flesh on it, of course).' If you want to write a good sex scene, burning Cosmo is the first step.

- Most importantly, and worst of all, there are absolutely no sensible words in the English language to describe the important body parts. What are you going to call them? Penis? You sound clinical. Cock? You're swearing, which sounds brutal, especially if the characters are not usually this forthright in their behaviour. Manhood? It's time to go and stick your head in a bucket of cold water. And let's not even consider the difficulties of describing the female bits. Given human nature, it's pretty much impossible to invent a word for a sexual body part that's neutral in the same way that 'arm' or 'neck' are neutral; people get all giggly and before you know it, the word has its own register.

This last has given rise - oh wait, now there's another one:

- Unintentional innuendos rear their heads from every corner of the page. If you want an opportunity to look stupid, a sex scene will give you one. And once people start noticing them, everything you write starts to look accidentally naughty, and then you're stuffed. I'd continue in this vein, but I think you've got the point, so I needn't keep it up all night.

Ahem. The difficulty of naming body parts has given rise to my basic rule of thumb when it comes to sex scenes: don't mention his willy. In the absence of a sensible word to use, it's better to say nothing. The trouble with the no-neutral-words issue is that, while your lover's breast, penis or clitoris is firmly attached to your lover, an integral part of their body, it's almost impossible for the words naming them to be integral parts of the sentence. They leap off the page, jarring the register; it's as if the text speaks at a normal volume throughout and then the occasional word shouts off the page, which does not convey the sense of physical harmony that good sex is all about. It would be like having a movie sex scene where romantic music plays in the background, and then a raucous car horn honks every time one of the actors touched a sensitive part of the other. Not good.

There are other ways of describing sexual encounters that don't involve giving the precise details of what happened, and very often, that's all you need to know. Sex, after all, is an event in the plot; it matters because of the effect it has on the characters. Hence, you can probably get away with telling the reader how the evening went in general terms, mentioning only the relevant things - whether she enjoyed herself, whether he was considerate or rough, whether she managed to stop thinking about her ex for the duration, whether he felt pleased with his performance or embarrassed afterwards. You can get out of writing the scene by writing elliptically and sticking to the after-effects.

There are, of course, other possibilities. If you're very clear in your mind about the effect the sex is going to have on the characters, you can slant your emotional tone in that direction. This is probably more the case if you want the sex scene to describe bad sex, because sex scenes often make readers feel uncomfortable, and if you want to convey an uncomfortable encounter, you can just take that discomfort and go with it.

There is also, of course, the choice of just writing porn, but you will have to deal with the fact that, if it's not a porn novel, the scene will be different in feel from the rest of the narrative. If it's too different, readers may actually skip it if they're not in the mood for porn.

One way to look at it is character writing. A subjective sex scene can be much easier. For instance, you can get around the don't-mention-his-willy rule if you're writing this scene from the point of view of Gideon, who was raised by his religious maniac parents to think of his lady-pleaser as his 'Serpent', or Jack, who's a bit of a lad and says 'cock' whenever he can, or Sophie, who's never seen a naked man before and thinks 'penis' because it's the only word she can bring herself to say in conversation. If you can write from inside the characters' heads, that solves a lot. But if that's the case, don't forget to keep viewpoint consistent: if we're seeing it through Gideon's eyes, we're seeing it through his eyes, which means that he's also going to be more aware than most people of the creak of the bed, because the neighbours might hear and realise he's sinning; he'll be either bewildered or disapproving or relieved or secretly thrilled if the girl is bold, vengefully sadistic or guilty and awkward or gallantly tender if she's shy; his awareness of her body, and the words he'll think about it, will be conditioned by his religion and also by his ignorance, assuming this is his first encounter, so there'll be surprise at the feel of her flesh, an intense sense of observation that comes with discovering something new ... you get the idea. You can write a really good sex scene if you have the character's psychology down, but it takes a lot of empathic projection.

In a way, this difficulty is a good thing. Sex scenes force you to be inventive. Some literary problems, you can look at what other writers have done and think, 'Oh, that works', but with sex, it's what other writers have done that's the problem, or at least a part of it. So many pulp romance novels have used the word 'moan' that it immediately invokes a pulp romance novel. So many lad mags have used the word 'tit'; so few people at all have used the word 'frenulum', even if they see one every day. Other authors' prose has a baleful effect if you're writing a sex scene. The thing to do is try to remember why the sex is taking place. What's the importance of it to the story and the characters? And from there, you can decide whether it needs to be detailed or vague, and if it needs to be detailed, what kind of details best convey the mood. It's a tricky old job, but someone's gotta do it.