Eli5: If water can’t be compressed under normal conditions, then how does water pressure work?
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You don’t have to compress something to put it under pressure. Compression is what happens when you increase the pressure of something, pressure isn’t the result of something being compressed.
If what you are compressing is very squashy, like gas, then the pressure doesn’t change much as you change the volume a little bit. But with a fluid that is largely incompressible, the pressure will vary wildly under very small changes in volume. If you were to take a large steel tank, fill it with water under high pressure, seal it and remove the pump, only a tiny amount of water would flow out of the tank when you opened it, and most of that flow would come from the stretch in the steel tank, not the water’s compression. Fill a tank with air under pressure in the same way, and lots of air will flow out.
With the pressurised water systems like domestic water, the water isn’t under pressure because of compressed water – the pressure is either provided by gravity, by having a high water tower that provides pressure from gravity; or in smaller systems, a sealed tank is half filled with water and half with air, and the high-pressure but squashy air pushes down on the water, putting it under pressure. When you turn on the tap, water flows all the way from the water tower or the pressurised tank to your tap.
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