LEGO Working Padlock
Working, but not in a sense that it's lockable. I mean, let's face it, this is a LEGO lock, you can just pull it apart. It's working in a sense that you can open or close it. From a distance it even looks like the real thing. You can prank someone by putting it on their locker, and see how long it takes until they realize it's not a real lock.
Lock it, open it.
The part that "locks" it in place is a "Technic Axle 1L with Pin 2L with Friction Ridges" (11214). You might need to play around with different pieces to find a combination where the axle is more snug than the pin, so when you open the lock, it separates at the pin end, and not at the axle.