The easiest way to detect nuclear material is typically when it's already too late. By the time seismographs, infrasound sensors, and radiation readers pick up on a blast, it's already happened. Catching a nuclear weapon before it goes off is a lot trickier, which might be why the Department of Homeland Security (DHS) is looking at some rather unconventional ideas. The latest: a small, wearable radiation detector. Think of it like a FitBit, only instead of telling its user about calories burned, it lets them know if they're close to any nuclear weapons.