A thousand years ago, Vikings navigated with a sunstone, which they used to locate the sun on cloudy days. The stone—a calcite crystal called Iceland spar—funnels light into two beams. When the beams appear equally bright, the rock is facing the light, even if it’s obscured. Researchers now use calcite to funnel light around tiny objects for "invisibility" cloaks.
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.