In fact, in order to trade, you need to give something in exchange for even that silk. One of the Chinese motivations for trading with others is having access to horses. Horses were valuable not just for transporting goods, but also in war, especially fighting some of their northern enemies who had access to horses. They also were able to get gold from Europe, able to get cotton from India. In Europe, you also had other manufactured goods, so you had this fairly vast trading enterprise emerge between these different parts of the world. And to be clear, some of the things that were traded allowed or facilitated the trade itself. I mentioned horses were a source of transportation, but camels in particular were very powerful sources of transportation, especially across Central Asia and as you get into the Middle East and Africa, because you have these large swaths of desert.