`Is that what you've been sitting here thinking all this time?

'Yes.'

`I thought you'd been quiet.'

'I was trying to imagine what it would be like to be a blind man trying to live in a discotheque. Or several competing discotheques.'

`Well, it's worse than that, isn't it? Mark said. 'Dolphins rely on sound to see with.'

'All right, so it would be like a deaf man living in a discotheque.'

'Why?'

'All the stroboscopic lights and flares and mirrors and lasers and things. Constantly confusing information. After a day or two you'd become completely bewildered and disoriented and start to fall over the furniture.'

`Well, that's exactly what's happening, in fact. The dolphins are continually being hit by boats or mangled in their propellers or tangled in fishermen's nets. A dolphin's echolocation is usually good enough for it to find a small ring on the sea bed, so things must be pretty serious if it can't tell that it's about to be brained by a boat.

`Then, of course, there's all the sewage, the chemical and industrial waste and artificial fertiliser that's being washed into the Yangtze, poisoning the water and poisoning the fish.'

`So,' I said, 'what do you do if you are either half-blind, or half-deaf, living in a discotheque with a stroboscopic light show, where the sewers are overflowing, the ceiling and the fans keep crashing on your head and the food is bad?'

'I think I'd complain to the management.'

'They can't.'

'No. They have to wait for the management to notice.'

A little later I suggested that, as representatives of the management so to speak, perhaps we ought to try to hear what the Yangtze actually sounded like under the surface - to record it in fact. Unfortunately, since we'd only just thought of it, we didn't have an underwater microphone with us.

'Well, there's one thing we can do,' said Chris. 'There's a standard technique in the BBC for waterproofing a microphone in an emergency. What you do is you get the microphone and you stuff it inside a condom. Either of you got any condoms with you?'

'Er, no.'

'Nothing lurking in your sponge bags??

'No.

'Well, we'd better go shopping, then.'

By now I was beginning to think in sound pictures. There are two very distinctive sounds in China, three if you count Richard Clayderman.

The first is spitting. Everybody spits. Wherever you are you continually hear the sound: the long-drawn-out, sucking, hawking noise of mucus being gathered up into the mouth, followed by the hissing launch of the stuff through the air and, if you're lucky, the ping of it hitting a spittoon, of which there are many. Every room has at least one. In one hotel lobby I counted a dozen strategically placed in corners and alcoves. In the streets of Shanghai there is a plastic spittoon sunk into the pavement on every street corner, filled with cigarette ends, litter and thick, curling, bubbly mucus. You will also see many signs saying `No spitting', but since these are in English rather than Chinese, I suspect that they are of cosmetic value only. I was told that spitting in the street was actually an offence now, with a fine attached to it. If it were ever enforced I think the entire economy of China would tilt on its axis.

The other sound is the Chinese bicycle bell. There is only one type of bell, and it's made by the Seagull company, which also makes Chinese cameras. The cameras, I think, are not the world's best, but the bicycle bells may well be, as they are built for heavy use. They are big, solid, spinning chrome drums and have a great resounding chime to them which you hear ringing out through the streets continuously.

Everyone in China rides bicycles. Private cars are virtually unheard of, so the traffic in Shanghai consists of trolley-buses, taxis, vans, trucks and tidal waves of bicycles.

The first time you stand at a major intersection and watch, you are convinced that you are about to witness major carnage. Crowds of bicycles are converging on the intersection from all directions. Trucks and trolley-buses are already barrelling across it. Everyone is ringing a bell or sounding a horn and no one is showing any signs of stopping. At the moment of inevitable impact you close your eyes and wait for the horrendous crunch of mangled metal but, oddly, it never comes.

It seems impossible. You open your eyes. Several dozen bicycles and trucks have all passed straight through each other as if they were merely beams of light.

Next time you keep your eyes open and try to see how the trick's done; but however closely you watch you can't untangle the dancing, weaving patterns the bikes make as they seem to pass insubstantially through each other, all ringing their bells.

In the western world, to ring a bell or sound a horn is usually an aggressive thing to do. It carries a warning or an instruction: 'Get out of the way', 'Get a move on', or `What the hell kind of idiot are you anyway? If you hear a lot of horns blowing in a New York street you know that people are in a dangerous mood.

In China, you gradually realise, the sound means something else entirely. It doesn't mean, 'Get out of my way, asshole', it just means a cheerful 'Here I am'. Or rather it means, `Here I am here I am here I am here I am here I am...', because it is continuous.

It occurred to me as we threaded our way through the crowded, noisy streets looking for condoms, that perhaps Chinese cyclists also navigated by a form of echolocation.

'What do you think?' I asked Mark.

'I think you've been having some very strange ideas since we came to China.'

`Yes, but if you're weaving along in a pack of cyclists, and everyone's ringing their bells, you probably get a very clear spatial perception of where everybody is. You notice that none of them have lights on their bicycles?

'Yes...'

'I read somewhere that the writer James Fenton tried riding a bike with a light on it in China one night and the police stopped him and told him to take it off. They said, "How would it be if everyone went around with lights on their bicycles?" So I think they must navigate by sound. The other thing that's extraordinary about cyclists is their inner peace.'

`What?

'Well, I don't know what else it can be. It's the extraordinary, easy unconcern with which a cyclist will set off directly across the path of an oncoming bus. They just miss a collision which, let's face it, would not harm the bus very much, and though they only miss by about nine millimetres the cyclist doesn't appear even to notice.'

'What is there to notice? The bus missed him.'

'But only just.'

'But it missed him. That's the point. I think we get alarmed by close scrapes because they're an invasion of space as much as anything else. The Chinese don't worry about privacy or personal space. They probably think we're neurotic about it.'

The Friendship Store seemed like a promising place to buy condoms, but we had a certain amount of difficulty in getting the idea across. We passed from one counter to another in the large open-plan department store, which consists of many different individual booths, stalls and counters, but no one was able to help us.

We first started at the stalls which looked as if they sold medical supplies, but had no luck. By the time we had got to the stalls which sold bookends and chopsticks we knew we were on to a loser, but at least we found a young shop assistant who spoke English.

We tried to explain .to her what it was we wanted, but seemed to reach the limit of her vocabulary pretty quickly. I got out my notebook and drew a condom very carefully, including the little extra balloon on the end.

She frowned at it, but still didn't get the idea. She brought us a wooden spoon, a candle, a sort of paper knife and, surprisingly enough, a small porcelain model of the Eiffel Tower and then at last lapsed into a posture of defeat.