A water thief allows firefighters to break down one larger line into several smaller ones, each with independent control of water flow at the valve.
Another device, used in antiquity, was called a water thief or clepsydra. Carl Sagan described it in his book Cosmos as "... a brazen sphere with an open neck and small holes in the bottom, it is filled by immersing it in water. If you pull it out with the neck uncovered, the water pours out of the holes, making a small shower. But if you pull it out properly, with the neck covered, the water is retained in the sphere until you lift your thumb."