The Venturi effect is the creation of a partial vacuum when the flow of a fluid is restricted, increasing its speed of flow. Thus, a pipe with a fluid flowing through it which narrows at one point before widening again draws fluid from a second fluid-filled pipe attached to the narrow portion. This occurs because, as the fluid's kinetic energy increases, its pressure must decrease.
The Venturi effect relies on the natural law that energy can neither be created nor destroyed. When a fluid speeds up because the pipe it is traveling through narrows, this increase in speed, on its own, constitutes an increase in kinetic energy. However, the pressure of the fluid actually constitutes a sort of energy, itself, the energy density of the fluid. The Venturi effect is a manifestation of the Bernoulli principle, the latter being a general statement of the lowered pressure of fluids when accelerated by a narrowed channel.
One example of the use of the Venturi effect is in aquariums, where it is used to aerate water. A pipe, open to the air, is attached at the other end to a pipe filled with water pumped from the aquarium. The water pipe narrows around the attachment point, causing air to be drawn into the water flow, oxygenating the water.