A Venturi tube demonstrates the principles of fluid dynamics and pressure. As air or water flows through a tube that narrows and then widens, manometers measure the pressure at various points along the tube. The fluid velocity increases in the narrow section, causing a decrease in pressure, which is indicated by the manometers. This demonstrates the Venturi effect, where fluid pressure decreases as fluid speed increases in a constricted section of the tube, illustrating the relationship between pressure, velocity, and cross-sectional area in fluid flow.