Herzog talks of how Ai Weiwei encouraged them to develop the "crazy, chaotic" structure of the stadium as far as it would go. He sketched a tree and a bird's nest to show that it would chime in with Chinese culture, would not be seen as alien. This is exactly as such collaborations should be. Who did what? If you can tell, then it isn't a real meeting of minds. It becomes just a commissioned artwork, fitting in with the architecture somewhere or other. That is emphatically not what is happening here. Something altogether subtler is taking place.

I like the sound of Ai Weiwei. Occasionally and fleetingly, he appears in photographs. For instance, he is photographed dropping the Han jar, just so that we know the act really happened. A thickset, jowly, rather surly-looking man. The jar was dropped in 1995. He has smashed other ancient artifacts. He has also presented, as priceless objects, modern fake copies of such relics. All the craft skills are still there in China - you can only tell what is original and what is new by carbon dating. So Ai Weiwei is questioning notions of authenticity and value when he does this kind of thing.

He challenges the symbols of political and national authority and cultural imperialism, too, as in his series of photographs where he literally gives the finger to buildings as diverse as the White House, the Eiffel Tower, the Reichstag, a huge cruise liner, and of course Tiananmen Square. For good measure he has another photo of Tiananmen Square, dating from 1994, where a girl casually flashes her knickers at the camera as crowds stroll by, and a giant portrait of Mao stares over her shoulder.

What else can one say about Ai Weiwei? Plenty. He collects and exhibits feet and hands. They are fragments of stone sculptures from the Northern Qi period, again more than 2,000 years ago. They are the remnants of the destruction that occurred at the end of the dynasty. The heads of the statues were much prized and kept, the hands and feet were discarded. Weiwei has rescued and re-dignified them.