Gamemode: works in both modes
Requirements: head and water
If you got a head and place this in a water pipe of something else the water wouldn't go. It's like the cutted water think it's connected to the water source but it isn't. You can also replace the water source with a head, the water wouldn't still go.
But only if the water is completly spreaded otherwise the watersource would be updated and would flush away the head.