As a child growing up in northern China, Yan Xiao loved flying kites. A born engineer, he made them himself out of paper sails and plain bamboo frames. The kites were durable and cheap. Xiao left China at age 22 to study civil engineering in Japan and the U.S. but returned as a visiting professor at Hunan University in 2002. On a trip to the region’s vast bamboo forests, the memory of those kites gave the 47-year-old Xiao a flash of inspiration: Bamboo was strong enough for kites, but he suspected that it could be fortified to make even sturdier things, like bridges and houses.