This purifier’s immersion sensor is extra-sensitive. Others use a current to detect water before turning on, but this one tracks the reflection of an LED to work better in purer, less-conductive water like freshly melted snow. $100
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.