When is something published? This is a pretty straightforward question in newspaper terms - when an edition rolls off the press and is put in the hands of a third party. For the internet, the moment of publication is when you make a story available on your website.
More difficult to define is, when does an article expire? If, like Guardian Unlimited, you keep all your content in the same place and free to the world, the legal and technical definition of when something is published is "all the time". This leads to an interesting phenomenon, the equivalent of back issues being passed around ad infinitum, and discussed in a "hey - look at this" way. As it is possible to track how many times a story has been looked at through what we call page impressions (someone opening a page and looking at a story constitutes one page impression), we know exactly how popular any of our stories are at any given time - which can be sobering for journalists.
This week, one story refused to budge from the list of top stories. The second most popular story of the week originated on September 25 in the Observer, under the headline "Armed and dangerous - Flipper the firing dolphin let loose by Katrina".
As Mark Townsend wrote in his introductory paragraph: "It may be the oddest tale to emerge from the aftermath of Hurricane Katrina. Armed dolphins, trained by the US military to shoot terrorists and pinpoint spies underwater, may be missing in the Gulf of Mexico."
On the Sunday Flipper appeared in the paper the story won 25,000 impressions; on Monday it attracted a further 484,000; and to date it has picked up 915,000. Once you put a story into cyberspace it acquires a life of its own, well beyond the moment of publication. Flipper was Drudged - that is, he appeared on the US site the Drudge Report, which points people towards the quirkiest and most scurrilous stories on the web. From there one blog after another picks the curious item up and passes it on until the cumulative effect is as great as a major breaking news story. The Observer archive has a habit of doing this - in 2003, when war in Iraq was dominating news on the web, the biggest story on the Observer site was an article about a New York fish market where two mongers claimed a fish had started shouting in Yiddish as it was about to be beheaded.
On our education site, a story earlier this year gloried in the headline "Necrophilia among ducks ruffles research feathers". Like the dolphin, it became our most popular story for days on end. This may demonstrate the effect Google has on any story carrying a word with sexual connotations in the headline, or indeed popular interest in animals doing strange things. But it also illustrates a key principle of web distribution - the value of a "long tail".
If you publish all your material continuously, although the top story of the day will still attract a high degree of interest, other stories being talked about elsewhere on the web can draw an equivalent or even bigger audience over time. A third of our traffic on Guardian Unlimited comes from stories more than a month old. Hardly surprising as we have 3m items in the archive, all of which attract fractional traffic on an individual basis, but collectively make a considerable impact. So when people ask me why we don't charge for the archive or at least put it behind a registration wall, this is a powerful reason why not. We would rather be swimming with the dolphins - armed or otherwise.
· Emily Bell is editor-in-chief of Guardian Unlimited