Fluids are subject to 2 conservation laws: conservation of mass and conservation of energy. For a liquid, whose density cannot change signi¿cantly, conservation of mass requires high-flow velocities in which the liquid channel is constricted. Fluid pressure represents a form of internal energy, and therefore energy conservation requires low pressure where fluid velocity is high, and vice versa. This relationship between pressure and velocity results in many practical and sometimes counter-intuitive phenomena collectively called the Bernoulli theorem. Fluids also exhibit a friction-like force called viscosity.