You know how airport security will occasionally ask you to turn on a phone or laptop to prove that it isn't hiding explosives? Well, that's no longer just a rare inconvenience -- if you take certain flights, it's mandatory. The TSA now requires that you power on your gadgets when flying to the US from "certain overseas airports." If you have a dead battery, you're out of luck. You'll likely have to leave that hardware behind, and you might go through "additional screening" at the same time.