Like many sociologists I am taken with the idea of mobility and flow - of things and people on global trajectories winging their way from place to place. I teach a couple of sessions on a photography and urban cultures MA course and one day after class a student came to show me his photos. They were of discarded flip-flops washed up on beaches in Vietnam. He had piled them up on a beach; he had enlisted kids to help him collect an enormous heap of them. He said he liked everyday objects and wanted to photograph and write about them for his dissertation. A lot of students show me their photos. I thought: traces of lives, the imprint of feet, globalisation's plastic rubbish. What is plastic's social fabric? I thought no more of it.

Some time later, I was in Mozambique. I was supposed to be in Durban for the International Sociological Association conference: I did attend but I couldn't stand being in South Africa. It's not just that the city authorities cleared away the street children for the conference, or that they bussed participants from hotel to conference centre and asked that we not walk the streets for any reason (a couple of participants robbed at knifepoint underscored their warning). It's that I ended up staying in "safe" white ghettos where the inmates presumed I shared their racial politics and their fear. Is apartheid actually over? Social change is slow.

So I moved to Mozambique. It's fabulous: all red earth and prosperous-looking villages growing crops. There were shiny, newly surfaced roads but hardly any traffic. Instead, people walked. They walked miles in flip-flops with heavy loads on their heads. Gaggles of children walked five miles between home and school in flip-flops. Instead of the smooth swoosh of tyre rubber, these roads took the flap, flap of flip-flops - the people's transport; toes clenched to plastic soles. It struck me that shoes were really important in flow; especially these shoes. It also struck me that "flow" may not be the right word. People don't flow, they walk and stumble and lurch and bump into things. Flow suggests an easy smoothness. Later I learnt that flip-flops are the world's highest-selling shoe. When the world's population rises, so do flip-flop sales. I'd found a demographically sensitive object worn by millions of people who lack alternative footwear. A billion people worldwide still walk barefoot, so flip-flops, retailing for as little as 40 cents, are a first step up.

An idea was gathering - eureka academic-style - in slow motion. I was back at my desk where everything felt remotely possible. I was reading about the social lives of objects, the ways in which we live with them and what we can learn about the way societies work by studying them. I hit on anthropologist Igor Kopytoff's biography of a car in Africa. By tracing how the car was acquired, where the money to buy it came from, how it was used, for what and by whom, he pieced together how money circulated and how social connections were formed and maintained. I seized and reformatted the idea in topographical terms. What if I logged not just the biography but the journeys and circuits of a pair of flip-flops?

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Many flip-flop journeys begin in Fujian Province in southeastern China, where a large proportion of the world's flip-flops are made. They end as brightly coloured debris in the garbage dumps of cities in the global south. I was still at my desk. Maybe I could get into some factories in China and persuade the student - Michael Tan, who by this time was an assistant professor teaching art in Singapore - and his camera to come with me. Not only did he like photographing old plastic shoes, he also spoke Chinese. Then we could follow the onward journey of the flip-flops along the highways and sea routes of the global-distribution system to one of their biggest markets - Ethiopia.

Ethiopia has a high population (72 million plus) and a low average income, making it an ideal customer for cheap Chinese goods. Not only would it be possible to trace one of the zillions of tiny circuits composing globalisation, we would also capture those that pass through China with its phenomenal economic growth. We could explore the human fabric of globalisation, the connections between strangers, and catch a glimpse of China's relationship with Africa while we were at it. An idea was born and funded.

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These research plans made perfect sense from the vantage point of my desk in London. Once I started emailing what I thought were factory owners in China, the implausibility of my plans became clear. I got mostly one of two replies: "How many containers of flip-flops would you like to order?" or "No way are we letting Westerners into our factories while you are busy writing horror stories of child labour and human-rights violations in your press in the run-up to the Olympics. Fuck off!"

Except for one, and you only need one. Xiuchan - "call me Bill" - seemed to have a factory and contacts with a number of others he offered to get us into. He wrote long emails in beautiful formal English and made a hotel booking for us in the city of Fuzhou near the factories.

When I got there - I went ahead of Michael on the grounds that ferreting out information takes longer than taking photos - Bill met me and took me to the building featured in the glossy photos on his website. It was not a factory; it was a showroom where 3,000 different styles of flip-flops were displayed. Where to begin? Bill wasn't a factory owner; he was a broker who had been to business school and dealt with foreign customers in perfect English with immaculate cultural sensitivity. He took orders and commissioned production from his factory network. He offered to drive me to them. I was excited.

Over the next few days we visited factories with different scales and technologies of production; some made 10 million pairs of flip-flops a year. I interviewed bosses and watched the workers in action. There was no evidence of child labour, conditions were good and everyone seemed quite cheerful. In each factory I was treated to the legendary Chinese hospitality of the endless banquet. This was research where you gained weight.

Bill was really helpful but I couldn't focus on the kind of micro-study I had in mind while he was shepherding me around. I explained that I needed time in the factory to watch the production, learn about the different processes and talk to the workers. Bill pointed out that I don't speak Chinese. I suggested that as Michael would be arriving soon he could translate. Bill was not convinced. He would need to see if Michael was "suitable". Rattling around industrial villages, Bill explained, could be hazardous. I pointed out that Michael had extended family in Fujian Province. Would Bill consider him a suitable Chinese man for the task in hand?

Michael passed the Chinese-man test and we got to hang around all day in one of the factories. We interviewed the workers about their lives and their work. We discovered that they were migrants from the countryside. We learnt about what anthropologists call the "chains of unsettlement" that led them to the factories and how they got on with "locals". Michael took photos and I did drawings of factory layouts and work patterns. I was really excited when Michael discovered two makeshift homes inside the factory itself, where migrant workers lived with the continuous (24-hour) sound and fumes of production. We interviewed them in their homes when they came off shift. Things were working out nicely and when we came back the next day the boss wasn't there so we set to work again. The same the following day, and the next, too: no boss and free rein in the factory. The workers no longer posed for the camera: they were used to having us around. The data were getting richer and richer.

And then the boss returned and threw us out. We were interfering with production; we might be spies who wanted to set up a factory in competition. We had hit a Chinese wall. People speak openly and freely, and then they start to worry about what you will do with what they have told you and whether it will get them into trouble with the authorities.

Recognising that we could get Bill, the boss and the workers into a lot of trouble, we left. But being relentless researchers, we stationed ourselves outside the factory gates. We knew the workers well enough to get invited to their homes to do more interviews privately, so no one felt too exposed.

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Each day Bill would call to see what we had been doing.

"Did you stay in the hotel?" he would ask hopefully.

"No," I would have to admit, telling him we had been interviewing people in the industrial villages. I could feel him wince.

Meanwhile, there was something I could do for him in this asymmetric relationship in which he had been so generous with his time and contacts. He asked if I would give a talk at his old business school. The students learnt business English or Japanese and it would be good to have a talk from a native English speaker. Of course I agreed. In the car on the way I asked the head of department what she wanted me to talk about.

"Could you talk about British culture?" she replied.

"Yes," I said hesitantly (it's not my area). "How long would you like me to talk for?" I was assuming it would be 20 minutes at most.

"Oh, about two hours," she replied.

I tried not to look as anxious as I felt and hoped it would be a small audience so we could interact a bit. When I got there, 2,000 students were gathered in a lecture theatre and I got a standing ovation as I entered the room. Nothing I could say would deserve it.

I launched myself at the topic, but they were restless. The microphone wasn't very good so the students in the back half of the auditorium couldn't hear. Anyway, they were just learning English and in my nervousness I spoke too quickly. The lecture wasn't well pitched either and passed right over their heads. I ploughed on for the best part of an hour (as I am programmed to do) before coming to an exhausted halt. I wanted to go back to the hotel and hide in my room, but there was another banquet with the college principal to eat my way through first.

I sat next to the principal, my gracious host. He seemed to neither know nor care about the disastrous lecture; it was being there that mattered. He told me that his father was an illiterate farmer who still lived in a village. It dawned on me that the most significant thing about Chinese society and living in a country with such a rapid pace of social change was the breathtaking rate of social mobility and the routes - through education and contacts with party officials - by which people negotiated it. A middle class swam before my eyes - Bill, the principal, the factory owners and others, too - the plastic middle class.

There was not one eureka moment, but many, that led to my interest in researching flip-flops and thinking about plastic's human fabric in China. There were other moments too as the research moved on to Ethiopia, through the territories of pirates, warlords and smugglers. But that's another story.

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Caroline Knowles is professor of sociology and director of the Centre for Urban and Community Research, Goldsmiths, University of London.