I came across this DIY water saving device today and am so confused about how it works. So people just drill some tiny holes into a cap of a plastic bottle, and then insert it into the opening of a water tap. They say this can reduce water flow. But I don't quite understand why. After inserting the bottle cap, the surface area for water to go out reduces. But according to Bernoulli's principle, the volume of water going into the tap should be equal to the volume of water going out of the tap. So at the end of the tap, the water pressure decreases and the water speed increases, resulting in the same amount of water volume. So I think in the same time interval, the volume of water before and after using the bottle cap should be the same?