Eli5: If water can’t be compressed under normal conditions, then how does water pressure work?
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Sorta like asking how does sandblasting work if sand isn’t compressible. Air pushes on it. For a pressure washer, compressed air pushes on the water. For tap water, standard atmosphere and the weight of the water itself pushes the water out. Compression doesn’t really come into it at all.
Others have already answered the question: water _is_ compressible, and yet this is mostly unrelated to pressure.
But let me give you a very simple proof that water can be compressed: it conducts sound as everyone can test in a bathtub or lake. Sound is produced by small differences in pressure that propagate as a wave. Anything that cannot be compressed cannot conduct sound. And indeed, even other types of compression such as pushing on a stick or pumping water into one end of a pipe are moving through the system at its speed of sound!
However, there are no proper substances that truly are incompressible; as more abstract examples, vacuum and black holes could maybe be used, both of which indeed do not compress nor conduct sound.
There is a difference between compression and pressure. Some things compress with pressure–like a sponge when you squeeze it (compress it) with your hand. Other things (like a rock) not so much. Water is more like the rock–you can apply all sorts of pressure to it, and the volume won’t change.
A block of steel is pretty incompressible. But if there is one pushing you into a wall, you will feel the pressure.
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