Let's say you have a stream of water leaking out of an elevated barrel, and we ignore the effects of air resistance and assume water is incompressible. So, the lower (and faster) the waterstream falls, the more it will contract. Sure, you say, that's because of the chemical forces between the H2O molecules. And that's pretty reasonable. However, according to conservation of mass, if any fluid was leaking out of that tank, including ones where the forces between the molecules are very small, it should contract at the same pace as the water! Since the volume flow must be constant, Q=V*A, the further it falls the higher the velocity must become and thus the smaller the area of the stream must become - completely independent of what kind of fluid it is . So what forces drive this contraction? I assume the reason behind this phenomena is that the pressure in fluidstream decreases, but why? According to the Bernoulli equation, shouldn't the kinetic energy gained by the water particles be simply a result of their decreasing potential energy? Why would the pressure be converted into kinetic energy?