The basic characteristic of fluids is that they do not retain their shape. They flow easily under the influence of relatively low forces. In simple cases, typical velocities of liquids are near a few meters per second when subjected to small pressure differences. Gases, which have much lower viscosities than liquids, will flow typically with velocities of tens to hundreds of meters per second at relatively small pressure differences. Accelerations and decelerations of gases in various flow conditions are important in understanding the pumping mechanisms of jet pumps, flow through valves, and the general flow patterns of process gases in vacuum chambers. Fluid flow can assume several different modes, which have significantly different relationships between various physical parameters. The basic flow types are turbulent, choked, laminar, molecular flow, and permeation. Choked flow occurs when sonic velocity is reached in a flow restriction between two pressure regions.