The world of Pakistan's brick kilns...relies on the labor of migrant families, from girls of 6 to grizzled grandfathers... Brickmakers toil near the bottom of Pakistan's economic and social ladder...the great majority are bound to the kilns by debt...At least 200,000 Pakistanis, many of them children, work in more than 2,500 kilns across the country, according to studies by labor advocacy organizations...Human rights groups have exposed cases of kiln owners chaining or imprisoning workers.