How did the Silk Road carry ideas, not just silk?

By relay. The Silk Road was never one road but a web of trade routes, over 6,400 kilometres across Asia, active from the 2nd century BCE to the mid-1400s. Few people ever travelled the whole thing. Instead, goods, and the ideas riding with them, were passed from one middleman to the next, city by city, and societies along the way mixed what they received with what they already had.

Religion travelled this way. Buddhism entered China along these routes in the 1st or 2nd century CE, and the first people to translate its scriptures into Chinese were foreigners: Parthians, Kushans, Sogdians and Kucheans. The earliest known translator, An Shigao, was a Parthian prince.

Technology travelled too. Papermaking, first documented in China, spread to the Islamic world in the 8th century and reached Europe by the 11th. A popular story says Chinese prisoners taken at the Battle of Talas in 751 taught Samarkand to make paper, but scholars doubt it: paper was already used there decades earlier. According to one ancient historian, even the secret of silk was smuggled west, by monks sent as spies to steal silkworm eggs.

Not everything that travelled was welcome. Plague moved along the routes as well, possibly contributing to the Black Death. The name "Silk Road" itself is modern, popularized by a German geographer in 1877. But the idea behind it is ancient: wherever people trade, they also swap beliefs, skills and germs.