StellarNewton's Laws
At its core, a fluid can just be described as a bunch of extremely small solids, which combine into 1 macroscopic entity, a fluid. This also means that all of Newton’s Laws can also be applied to fluids. Since pressure exists all throughout, this means that forces are also exerted within the fluid. However, Newton’s 3rd Law tells us that these internal forces will cancel out, meaning that in a fluid, only external forces really need to be considered.