In 1736 the Swiss mathematician Leonhard Euler ended a debate among the citizens of Königsberg, Prussia, by drawing a graph. The Pregel River divided the city, now Kaliningrad, Russia, into four sections. Seven bridges connected them. Could a person cross all seven without walking over the same one twice?
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.