We just learned about the Valve Trim.
There are a lot of different kinds of valves.
One type of valve that is used a lot is the Ball Valve.
It's called a ball valve because inside the valve there is a ball.
The ball has a hole in the middle of it, and the valve handle turns the ball.
When the ball is turned one way, the water or gas can go through the hole in the ball.
When the ball is turned the other way, the ball blocks the water or gas, and keeps the valve closed.
(from: wikipedia - ball valve)
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