Cheap labour has built China’s economic miracle. Its manufacturing workers toil for a small fraction of the cost of their American or German competitors. At the bottom of the heap, a “floating population” of about 130m migrants work in China’s boomtowns, taking home 1,348 yuan a month on average last year.

That is a mere $197, little more than one-twentieth of the average monthly wage in America. But it is 17% more than the year before. As China’s economy has bounced back, wages have followed suit. On the coasts, where its exporting factories are clustered, bosses are short of workers, and workers short of patience. A spate of strikes has thrown a spanner into the workshop of the world.

The hands of China’s workers have been strengthened by a new labour law, introduced in 2008, and by the more fundamental laws of demand and supply (see article). Workers are becoming harder to find and to keep. The country’s villages still contain perhaps 70m potential migrants. Other rural folk might be willing to work closer to home in the growing number of factories moving inland.

But the supply of strong backs and nimble fingers is not infinite, even in China. The number of 15- to 29-year-olds will fall sharply from next year. And although their wages are increasing, their aspirations are rising even faster. They seem less willing to “eat bitterness”, as the Chinese put it, without complaint.