If you've been in Silicon Valley, Pittsburgh, Boston, San Francisco, or any of the other cities where autonomous cars are crawling the streets in a 21st century version of drivers ed, maybe you've wondered: What's up with that overgrown gumdrop-looking spinning thing on the roof? That, dear carbon-based life form, is lidar, perhaps the most important piece of hardware in the race to unlock self-driving cars for everybody. Lidar works much like radar, but instead of sending out radio waves it emits pulses of infrared light--aka lasers invisible to the human eye--and measures how long they take to come back after hitting nearby objects. It does this millions of times a second, then compiles the results into a so-called point cloud, which works like a 3-D map of the world in real time--a map so detailed it can be used not just to spot objects but to identify them. Once it can identify objects, the car's computer can predict how they will behave, and thus how it should drive.