Wheels are part of the pipe
that will turn on and off the water. To turn on the wheel you press and hold the wheel. When you let go then the wheel stops and whatever was coming out will stop.
Purple: Poison Water
Green: Toxic Ooze
When a wheel is present then there will be no entrance.
In Change is Good there are some changing wheels.
Sometimes the wheels are attached to a pipe with an entrance. In this case the wheel will be gray. It works when a fluid touches the entrance, and it will not go through unless you touch the wheel.