"You must know him," says Yang Weidong, pointing to a smiling face among many pictures of migrant workers, "He was beaten to death in a detention center for failing to provide urban temporary residence permit...I always feel he died for us."

He was talking about Sun Zhigang at a private museum on migrant workers about 40 km from downtown Beijing. Sun's death in 2003 prompted the abolishment of a law to detain and send migrant workers or homeless or jobless persons back home when they were caught without residence permits in cities.

Yang, 28, served as a voluntary guide at the exhibition "Migrant workers - 30 decades - Flowing history" at the museum. The museum was funded by a charity organization Oxfam Hongkong and set up by a group of migrant workers. It keeps about 500 items in the 200-square-meter display area.

Urban residents might be amazed at the colorful permits: temporary residence permits, work permits or employment certificates. "They would ask what were these used for?" Yang says, "Only we know these were the must-haves for us to move around in cities without annoying police."

Six years ago, Yang and his fellows hid themselves in a dark room in Shenzhen, a booming city in Guangdong Province. Police were clearing migrant workers without urban temporary residence permits. The three from Henan Province put down curtains and asked their neighbors to lock them inside so that police might think nobody was in.

They heard knocking at door in the afternoon and dared not let out noise. Their dog Yellow jumped out from under the bed and before it barked, they covered it with quilts.

When the knocking ceased, they found the dog was dead. It later proved to be their friends at the door. Yang told China Youth Daily he often felt sorry for Yellow and often mentioned Yellow to visitors in the museum.

Yang's experience was not rare for migrant workers to hide away from police when they didn't have temporary residence permits for either the fees were too expensive or they did not have enough documents to apply for one. The fees was 180 yuan (25.71 U.S. dollars) in Beijing in the beginning and it costs only five yuan now to get a permit, says 31-year-old Wang Dezhi, one founder of the museum.

Now Yang feels safe when walking in any city after the country abolished the law to detain and send back migrant workers without temporary residence permits that had been enforced since 1982.

Yang epitomize Chinese migrant workers since China's reforms and opening-up in 1978. The country reported more than 200 million migrant workers in 2007. China has been improving rules and laws to cope with the new changes and ensure migrant workers' rights. Migrant workers also come into public attention, for example, more popular films depict life of migrant workers.