Here's another financial problem for cinemas that rebounds on audiences: the intersection between concession stands and film length. It's generally remarked that cinemas depend on concession stands now to keep their profits healthy; even the great old Cambridge Arts Cinema, to my sorrow, closed down its original premises several years ago (which is now part of an enormous Borders), and moved down the street. The new place is fine, but it's notable that it has a cafe attached. A nice cafe, selling gastropub-style sandwiches and nibbles and alcoholic drinks, a pleasant place to eat before the film - but still, financially, more or less an upmarket concession stand. If customers aren't coming in big droves, you need them to spend as much as possible while they're actually there, and charging a high mark-up on inexpensive food is one way of doing it.

Now, this would be fair enough, if a bit gouging, if that was all there was to it. But there's a problem with that: films are getting longer - and unlike older long films, have done away with the intermission. Alfred Hitchcock famously remarked that 'The length of a film should be directly related to the endurance of the human bladder,' and mundane though it sounds, it's an excellent point. However good the film, it's very hard to concentrate if you're in pain. 'How long can I go without wetting my pants?' might not be the deepest question in the world, but when it applies to you personally, it will probably seem more compelling that whatever artistic or philosophical dilemma the characters are wrestling with up on the screen.

In Boogie Nights, porn auteur Jack dreams of making a skin flick so exciting that it'll keep customers in their seats post-orgasm: '...When they spurt out that joy juice, they just got to sit in it until they find out how it ends.' If that sounds ambitious, try stacking up the 5-odd millitres of an ejaculation against the 400-600 ml capacity of a human bladder: now there's a problem that can't be addressed with a discreet tissue and some baby wipes.

Worse than that, by straining the audience's loyalties between the story and their comfort, you actually create a needless acid test: I can think of at least one film where the best way to express my distate for it was to remark that I hadn't minded taking a bathroom break in the middle. An audience member debating whether or not to nip to the loos and miss some of the story, in fact, is asking themselves at what point they're prepared to say, 'Oh, stuff this film,' - and that's not a question you should encourage audiences to consider any more than you absolutely have to.

Which is where the concession stand will take a hit. If the film lasts an hour and twenty minutes, a drink might seem like a pleasant accompaniment; if it lasts three hours and ten minutes, do you really want a pint of liquid running through your kidneys? Pretty much every time I go into a cinema, I go through the same thought process: It might be nice to take in some popcorn, but if I do that I'll get thirsty, and then I'll want a drink ... nah, I'd rather be able to concentrate on the ending, thanks.

Teenage boys are the main target of blockbuster movies for a lot of reasons, but I can't help wondering if one of them is simply that they're equipped with a functioning prostate and no uterus to, as it were, press for their attention a couple of hours into the film. And if that's the case, given the limited number of major features for anybody other than teenage boys, I intend to nail 95 theses to the bathroom door.