THE Long Now Foundation's 10,000 Year Clock (designed but seeking cash to build) aims to measure the passage of time over an epoch rather than an hour. It is part of the outfit's mission to lambast short-termism and encourage longer-term thinking. Randall Munroe, creator of the popular webcomic xkcd, seems to be playing the same game. On March 25th Mr Munroe posted a seemingly static cartoon titled, "Time". Move a mouse over the image, and the caption "Wait for it" appears.

Babbage did. Nothing happened. But on revisiting the site after thirty minutes, the image had changed. In the intervening weeks a new frame of an animation has appeared at the same interval. It documents the creation of elaborate and improbably tall sand castles by "Cueball" and "Megan", a couple who recur in Mr Munroe's strips. The castle is then supplemented by a wooden construction. To what end, no one yet knows, and Mr Munroe won't say. A complete sequence so far, with annotation, may be found at the fan site Explain xkcd.

Mr Munroe's interests are far-ranging, and he has posted ambitious efforts in the past. Recently a frame hid a vast, draggable world; a few years ago, he plotted the full timeline of major character interactions from "The Lord of the Rings" and a few other epics. These are beloved by geeks and teachers alike. On visiting a science teacher's classroom with his older son recently, your correspondent saw a large-format version of "Lakes and Oceans" posted on a wall. (Mr Munroe makes his work available under a Creative Commons copyright licence that allows reuse for non-commercial purposes so long as the work is attributed.)

The readers of Mr Munroe's strips are clever dicks and he has confounded their attempts to peer into the future. When a web browser loads the cartoon's page, the image of the comic is not loaded directly; rather when the cartoon graphic is requested, Mr Munroe's server tells the browser the current frame's file name, which it duly retrieves and displays. Every half-hour a script on his server changes that redirection to the next frame's name.

The files are not called "frame1", "frame2", and so on, but rather "0734ba60427cab503539c5e8be809688f0a6b56254248d60c51235a7b662182b", to cite one recent file name. These are randomly picked characters, much like a well-chosen password. Readers cannot guess the next file name in a sequence to figure out the ending. (Mr Munroe's server may also go one step further and not copy the next file into a publicly retrievable location until it's needed, as well.)

This instalment of the comic has tickled the fancy of enough people to get its own wiki that tracks progress and advances theories. It may be months, even years, before all will be revealed.