Cinthia Marcelle's The Family In Disorder is a perfect illustration of the beauty of entropy. If you haven't met this gem of an idea, it's the science that proves things get messier, and that tidying up takes more energy than untidying.

It's firmly in the realm of Conceptual Art, a genre with a reputation for being hard to understand and not always worth trying, but this exhibition has a clear narrative, and at its heart is one of those ideas so simple you wonder why you didn't think of it. A week before the exhibition launch, both of the upstairs gallery spaces were laid with identical pieces of black carpet, with a wall of materials built across the middle. In the Piper Gallery you can see this layout preserved. The wall stretches from one side to the other, creating a pristine area that no-one can enter. The carpet fits perfectly in the room. The Main Gallery being bigger, the carpet stands as an island in the middle, and in that critical week the artist technicians who usually put exhibitions up for the gallery were invited in, essentially to play with the materials. What you see is the result of their exploration.

There are two ways to meet the exhibition: come up the stairs by the shop, and you'll follow the path of the artist, starting with the materials, and seeing what they become. Or, come up from the café, plunge into the main feature, then travel through to find its origin myth. I went for the latter option, and so my first impression was of a giant ship's rigging, though quite a piratical ship, with velcro forming a barrier, and hundreds of pieces of masking tape wafting in the breeze.

In many ways what the main gallery looks like is not the point - this is conceptual art after all, so the journey is the important thing. And of course the artwork isn't "finished" - when time was up I'm guessing the artist-makers had to have materials prised from their fingers, and only the threat of the smoke bomb they were going to detonate over the whole lot got them out of there. If the artwork were to be recreated elsewhere, it would be a repeat of the experiment rather than a painstaking recreation of the finished creation. In addition, the exhibition will look slightly different every day - even if all visitors are careful not to knock into anything, the brick and chalk dust footprints will still spread and migrate as time goes on.

Having said that, the resulting creation is beautiful, especially in its detail, which can't help but reveal a bit of the personalities of its makers. Here is a small cairn of velcro and wrapped bricks, there a chalk henge, and in one place a huge hanging ball. As you're ducking out of its way it's impossible to tell (without breaking the rules and touching it) whether it's full of feathers or rocks. Forbidden from using tools, the makers were much more inventive with the materials. Shards of brick could cut masking tape and plastic, wooden poles and string could hoist other wooden poles over the high rafters. And the nominal 15 materials spiralled into many more, as the empty rolls from the centre of masking tape became a prized new instead of a waste product. Far from being natural disorder in the universe, you can see intelligent design, wit, and a Robinson Crusoe spirit at work. The Artist-creator and Artist-makers are separated, which seems to highlight the different parts of the process.

Poised in between the two halves of The Family In Disorder is Truth Or Dare, a video installation showing a spinning triangle, never still, shot in the harsh glare of the Johannesburg sun. It's a contradictory piece, with the video showing the triangle stationery, and a still photo rotating. In addition, if you are adventurous and follow the inviting warm light rather than the almost-closed door, you'll be rewarded with some tiny conversational drawings.

Back in the 1960s, Modern Art Oxford commissioned lots of site-specific works, and a good example tells you something about the space. Marcelle's work fits perfectly in the spaces here, setting up a tension between the cosy but isolated Piper Gallery, and the open, cluttered main gallery. One of the surprises is the relative size difference - the carpet shows clearly that the main gallery is not that much larger - even the gallery staff were fooled by the lofty ceilings into exaggerating that difference. There are other surprises, like the different appeals of the cold, clinical wall of materials, ripe with possibility but needing human interaction, compared with the warm wit of those same materials disordered. I conclude that's where family is to be found, excluded from a pristine space, but revelling in play and exploration. I head back to my untidy house with a newfound perspective.