Only a few generations ago, young urchins could wind down from a hard day working up the chimney by playing with toys made from mercury - or even with the metal itself. Mercury has great properties for entertaining kids: a liquid at room temperature, it expands and contracts according to temperature, and it can be divided into separate globules and recombined at will.

Unfortunately mercury is also hideously poisonous, something which limited the chances of its early admirers passing their mercury fascination onto their offspring. So in these safety conscious days, mercury remains firmly behind glass - although thanks to the cunning mind of Archer Maclean, that glass now includes the screen of the PSP. Employing many of the metal's unique properties into the form of an absorbing puzzle game, Archer Maclean's Mercury is as likely to mess up your fingers as playing treacle-ish marbles with the deadly metal, only now it's because you'll get RSI from cradling your PSP too obsessively.

Yes, be warned: Mercury is addictive. Even the later tutorial levels can be difficult and the game proper soon escalates to become infuriating. Yet like all great games, Mercury has 'one more go' appeal. That final attempt is soon your eleventh, and when at last you breakthrough to a new level it seems rude not to try it. The cycle continues...

So what does playing Mercury actually entail? Puzzle game reviews often include the phrase; 'it all sounds simple enough' but Mercury doesn't even sound simple. You control a blob of the liquid metal, which you guide about a 3D environment made up of various platforms, by tilting the entire level with the PSP's analog stick. Your blob can be sliced into pieces, recombined, resprayed, teleported, chopped, bashed, prodded and more. Worse, tilt too enthusiastically and you can lose some or all your mercury blob off the edge of the platforms. Lose too much and you're back to the start for One More Go.

Complicated enough? But Mercury then mixes up these elements to create fiendishly fiddly puzzles. You might have to control multiple blobs at once, flick switches, use conveyor belts, light beacons, and avoid your blob being eaten by little Pac-men-a-likes called Mercoids.

Oh, and you're doing all this against the clock.

Now for the good news: Mercury is simpler to understand in play then it is to explain. Not easier - not at all - but its brilliant recreation of the behaviour of a mercury blob makes it simple to pick up and get going. A typical level progression runs the gamut of 'What the ?$*?' when you start, to 'Ah' when you figure out how to complete the level, to 'Gotcha!' when you eventually do. And despite likely being on your twentieth 'last go' when you finally beat it, the incredible thing is even though you hated right up until you cracked it, you'll almost certainly retry the level straightaway to get a better score. That's addictive puzzle gaming at its finest.

It's important that Mercury gets that core gameplay so right, because elsewhere it's fairly standard. The graphics are commendably abstract and functional, but you won't be using them to show off the power of your PSP. The music and sound effects are strangely more-ish, but rather old-fashioned, as is the interface and indeed the whole Mercury experience, which is reminiscent of the old home computer games it resembles (the classic Marble Madness, in particular). There's no autosave, for instance, and it's all too easy to forget to record your progress in the euphoria of completing a few levels. The perfunctory multiplayer mode is barely worth the mention.

But none of this really matters, when the single-player game is so finely judged. It's a taxing long-term challenge - at the opposite end of the puzzle-playing spectrum from super-hip PSP rival Lumines. Lumines' is more fun to play, but Mercury is more satisfying to beat.

All that living on the edge, and it doesn't even kill you.

Archer Maclean's Mercury is on sale now.