A water thief is a special tool that firefighters use to take water from big hoses and put it into smaller ones. Imagine you have a big water hose that is as big as a basketball. But you need to put water into a really small hose that is only as big as a pencil. That sounds hard, right? But firefighters use a water thief to make it easier!
It's kind of like a magic wand with three nozzles. They attach the water thief to the big basketball-sized hose, and then they can put the smaller pencil-sized hose on one of the nozzles. Water comes out of the big hose and goes into the water thief. Then, it gets split up into three different streams and comes out of the nozzles. The firefighter can control how much water goes out of each nozzle to make sure the right amount of water gets to where it needs to go.
So, a water thief is a special tool that helps firefighters take water from big hoses and put it into smaller ones, making it easier and more efficient to fight fires.