Fischli and Weiss: A Retrospective Tate Modern, London SE1, until 1 January  Carsten Holler: Unilever Series Turbine Hall, Tate Modern, until 21 March  To say that Peter Fischli and David Weiss are the great comedians of contemporary art may not seem like the highest of praise. So much art, after all, seems unintentionally comic these days. But since they first started working together some 20 years ago, these Swiss wits have become a renowned double act, finding any number of ingenious ways to tickle the brain into quizzical - and sometimes serious - laughter.  Their installation for the opening of Tate Modern, for instance, was a colossal sculptural gag. Room 10, Untitled took the form of a gallery that clearly wasn't at all ready for viewing. Littered with paint cans, ladders, gaffer tape and fag-ends it looked as though the builders were still in, until you saw the familiar warning: Do Not Touch. Each object had in fact been carved by hand from polyurethane, ill-considered trifles laboriously rendered more substantial and weighty. Skits on the ready-made, these marvellous illusions also sent up the etiquettes of High Art - even when you knew they were sculptures, you still wanted to clear them up.  This installation is part of their current retrospective, along with all sorts of other objects: things (and their quiddity) being the stuff of life to these artists. Here you can see some very humdrum items - a wardrobe, a root, a bean - getting the earnest art-treatment, absurdly yet exactly reproduced in black rubber. The bean becomes as monumental as the wardrobe and both seem suddenly grave. I don't know why this is quite so funny - which is part of the pleasure in itself - though it seems the visual equivalent of deadpan humour.  Fiddling about with more gaffer tape, Fischli and Weiss come up with a hilarious figure of Ben Hur using an upturned bowl for a chariot: true Blue Peter versatility. A bottle of wine perilously balanced on a saw becomes a metaphor for Confidence and Ego. A boot, a rucksack and a chunk of cheese and you've got an Alpine Still Life  These foolish things can also be made magical through simple elevation. One of the most beguiling works here, Son et Lumière, features nothing but a green light refracted through a cheap plastic glass. The beam is beautiful enough, but its ever-changing radiance is achieved by revolving the glass on a hopelessly amateur turntable apparently botched from a cake-stand (the sound of the title being precisely that of thin plastic glancing and tumbling): a humble but compelling enchantment.  Until I saw this show I thought there must be more guile to Fischli and Weiss. But their intelligence is never cynical. Even when they go to the opposite extreme - towards comic deflation - it isn't with archness or spite. The centrepiece here is a room filled with little clay tableaux, celebrating overlooked moments in history: Mr and Mrs Einstein fast asleep after conceiving young Albert, two buffoons failing to invent the wheelbarrow, a crucial moment in the development of the steam engine which features, of course, a pot about to rattle off a stove with jets of steam all bathetically worked in thick clay.  These are comical, charming, helplessly brought down by the very choice of material from which so much of their humour derives. I specially liked a heroic attempt to show the breaking of a speed record. There are tiny landscapes and imaginary portraits - Plato, Nero enjoying the view of burning Rome - and several philosophical concepts quaintly embodied. Each feels like part of a series of idiosyncratic observations and thoughts that could run and run for as long as Fischli and Weiss are alive - which goes to the compendious nature of their work.  The spirit of curiosity has never died in these artists. They would like to point everything out. Naturally there would have to be an immense body of images to represent this whole big world of things we don't properly notice, and so there is. One work, cogently titled Visible World, simply amounts to slide after slide after slide.  And it's intriguing, enjoyable, on a quite uncomplicated level: all this looking at the overlooked. But is it really enough just to point and stare like children, when we are adults? My sense is that the artists would like it to be so, but know we must question what we see. What to make of their endless succession of airport photographs (is there actually anything to see when you look at these pictures?) And all these excessively beautiful flowers - what can you (and they) do but snap them, foolishly, with a camera?  Their masterpiece, and it's not too strong a word, is 30 minutes of sheer pleasure: a film as exhilarating for an adult as a child. It shows a series of objects - tyres, bottles, cup, candles, chairs - tipping and exploding, teetering and colliding, in a brilliantly inventive chain reaction.  Milk spills into a beaker that overflows, tipping a candle down a runway, igniting a tiny volcano of sawdust. Water melts a pillar of sugar cubes, toppling a load of burning rags that flare like a comet, causing a flash flood, and so on. It is a miraculous feat of physics from first to last, but each sequence is also beautifully designed as sculpture. And as you watch, it veers from slapstick to high comedy to darker intimations of disaster, pollution, even war.  What started as an innocent Heath-Robinson conceit, albeit without purpose, gradually escalates until the tension becomes almost overwhelming. Watching The Way Things Go you realise that a concatenation of mere objects has turned into a stupendous epic. I won't give away the ending.  Fischli and Weiss are not to be missed, but you can hardly avoid Carsten Holler. The Belgian-born artist's five slides spiral down the Turbine Hall like gigantic silver ringlets. He says they are art and not art, these already-famous fairground rides: but only a fanatic would find them more than artful. The G-force in the highest is frightening. Even the lowest sends you hurtling side to side. Slick as fireman's poles, undeniably exciting, they are simply the fastest exit from the pleasure dome.
Art: Twenty years of wit from the endlessly inventive Swiss duo Fischli and Weiss will have you chuckling - before you shoot off down one of Holler's slides, says Laura Cumming.