In a poor district in the southern city of Kunming, children play beside heaps of gravel left by construction crews.
The scene brings back painful memories for Li Qifang and his wife, Pu Caiju.
Pu Caiju has been keeping her son's toys safe for when he returns
More than two years ago their four-year-old son was playing outside their house when he was abducted. They have not seen him since.
"He disappeared on 15 March 2002. His father took him outside to play with his spinning top, and popped upstairs for two minutes. Suddenly the child wasn't there any more," said Pu Caiju.
"That day we couldn't imagine such a thing could happen. We were crazy with fear, searching the little alleys, big roads and bus stations. We searched for days, but didn't find anything," she said.
The couple now fear their son was abducted by a baby trafficking ring.
Buying and selling children is illegal in China, but child trafficking may be on the rise. A traditional preference for sons, combined with family planning restrictions limiting families to one child, have created a particularly strong market for boys.
Christian Voumard, the Unicef representative in Beijing, says rising prosperity is contributing to the problem.
"We heard that the price for a male child will be around $3,600. It's much higher than the price for girls, which will be between $120 and $1000," he said.
"As people have more money and more resources in south and west [China], then they could be tempted to buy these children."
No one knows how widespread child trafficking is, but Unicef and the Chinese government estimate 1,000 babies are bought and sold every year.
Those who have lost children suspect the real figure is much higher.
Li Qifang has been collecting a list of the children who have disappeared in Kunming. This list now has 200 names.
He has also started an informal support network for parents, which has begun liaising with similar groups elsewhere, like in the southern city of Dongguan.
He suspects trafficking rings are targeting poorer districts, where people cannot afford childcare.
For many parents, the loss of their children has led to a crisis of faith in the authorities and police.
All the couple have left are memories of their son
"Time and time again, the [police] have told us they're investigating," said Li Qifang. "After two years with no information or leads at all, how can we believe them?"
So he decided to take matters into his own hands.
He appeared on television to talk about his work, and this led to a startling telephone call.
The caller tipped him off about a young boy fitting his son's description, who had appeared about two years ago in a village in southern Guangdong province.
Mr Li was convinced there was a chance the boy could be his son.
"My contact says the child's been telling people his real parents live near the train station, and sell pork for a living. When my son went missing, I was living by the train station in Kunming, working as a butcher," he said.
The village where the boy was living is almost 2,000km from Kunming, but Mr Li decided he had to make the trip to see the child.
It was a long journey by train and bus, drawn out further by anticipation.
Our contact took us to the village under the cover of darkness, and pointed out the street where the little boy lived.
It was an unexceptional rural village, with small wooden three-storey houses lining a dusty street, and roosters crowing.
In the morning, we parked near the local primary school and watched the children going past.
Mr Li sat by the window, every muscle tense as he scrutinised the faces of passing students.
After three-quarters of an hour, he told me he had seen a boy who looked like his son.
Mr Li returned to the school as the children were let out for lunch.
"I saw that boy and thought he did look like my son, so I slowly followed him. The child saw me... and he kept turning around to look," he said.
"I followed him to one of the doors that our contact pointed out, and saw him going into the house."
It seemed positive. Mr Li was quietly exultant. He rang up the police station in Kunming, who put him in touch with the local police.
Days later, accompanied by a police officer, he returned to the village to look for the boy, first in official files and then at the school. This time the boy was not there.
"The child hadn't been registered with the police," he said, dismayed.
Families in Kunming have started an informal support network
"Then we went to the school gates to look for the child. He wasn't there. Maybe the person who bought him heard we were looking for the child and hid him."
Back home in Kunming, he had to break the news to his wife. First he told her he had found their son, then he had to tell her that the boy had vanished without a trace.
After two and a half years of waiting for news, she was heartbroken.
"I had great hope in this trip. I didn't think the results would be so disappointing. I'm really sad," she sobbed.
"I think the boy he saw was our son, but there's nothing we can do."
Mr Li and his wife say they have not yet given up hope. They are determined to go back in a few months to look for the same boy again.
But they know the chances of finding their son are miniscule.
Meanwhile the list of missing children continues to grow, as ever more desperate parents contact Mr Li with their own stories of loss.
All the parents say they will never give up looking for their children.
But most of them realise it is likely to be a search that lasts the rest of their lives.