Even concrete can float. Materials scientist Joseph-Louis Lambot built a concrete dinghy featured in the 1855 World's Fair. And to conserve steel during World War II, the U.S. made 24 concrete ships. Today, college teams race custom boats at an annual concrete canoe competition. A team from the University of Wisconsin, Madison, for example, used concrete mixed with tiny air-filled glass spheres to build a 20-foot-long vessel that supports a four-member crew.