According to the second Wikipedia derivation for incompressible fluids https://en.wikipedia.org/wiki/Bernoulli's_principle the change of the kinetic energy of the system equals the net work done on the system. Why only kinetic energy? Why can't the work done on the system also change the potential energy? (Note: I don't mean gravitational potential energy - here, we only consider the case where the elevation is the same for the whole system.) I'm thinking (correct me if I'm wrong) that in a region with higher pressure, there is an internal potential energy which is greater than in a region with low pressure. It seems logical that if that the harder the molecules in the fluid press against each other, the more work can be done on a neighbouring part of the fluid. If it wasn't so, how could work be done at all on the system by the pressure of the neighbouring fluid at the ends of the system? Where does the energy doing the work come from? And if there is such an internal potential energy, why doesn't it change when work is done on the system? Why does only the kinetic energy change?