Usually when you pull something out of your bag that deploys two prongs, you're about to tase somebody. But this here is a smartphone case, and one of the cooler ideas I've seen in that arena.
The PocketPlug smartphone case not only contains a battery, but features two flip-out prongs that can go right into an outlet, no cables necessary. You can leave the phone in the case while it charges, or pull it out to continue using it; either way juice is flowing into the case's battery.
If this project sounds familiar to you...maybe it is. The JuiceTank, as it was initially called, was successfully Kickstarted—back in May of 2012, when it was designed for the iPhone 4. Now it's 5s-compatible, or at least will be when you get it; it's currently up for pre-orders, at $64 a pop versus the $80 projected retail.
Here it is in action: