Anything else is simply too big a hassle. Companies like UPS and Amazon prize efficiency above all, and deploying a fleet of drones from a warehouse in the middle of nowhere wastes time. Making them fly all the way back wastes energy. And you still need trucks, because drones can't schlep more than a few pounds. But if you put the drones in the truck and fling them at houses to cover the last mile or so, well, then you're on to something. You're saving the driver the trouble of parking the truck, getting out, finding the package, and hoofing it to the door. Think of it as a paperboy riding his bike down the street, tossing the newspaper onto each porch.

UPS made a test run Monday in Tampa, Florida. Sid Perrin trundled through a rural neighborhood in a UPS van with an odd lump on the roof. Instead of taking a long driveway to a remote blueberry farm, she put the truck in park, climbed into the back, and placed a package in the belly of a drone. Back in the driver's seat, she tapped a command on a touch screen.  The roof of the truck retracted, the drone took flight, and Perrin continued up the road to her next destination.

The drone, meanwhile, flew a short distance to the house, deposited the package, and found the truck—where it plugged itself into a charger to await its next flight. And damned if it didn't work.