The great secret that China kept.
Silk was a part of the Silk Road trade. It would be a closely guarded secret on the new ways farmers would weave and dye silk. The penalty for revealing a secret would be death. The silk they made would be prized by wealthy Romans, and they were willing to pay high prices for it. The prices were high, because traders would pass on goods to another trader, and the prices would go up as the goods were passed on to each trader. More than silk was traded, though. Missionaries from India traveled to China and brought the religion of Buddhism. By the time the Han dynasty ended, Buddhism was becoming a major religion in China.