Used since the time of the ancient Egyptians as a method for transferring liquids from one vessel to another, the siphon is a dependable tool. Although, the act of siphoning beer from a fermentation barrel or wine from a demijohn is a skill that has been passed down from generation to generation, do we really know how the siphon works? It is widely believed that the siphon is principally driven by the force of atmospheric pressure. An experiment is described that shows that a siphon can function even under high-vacuum conditions. Molecular cohesion and gravity are shown to be contributing factors in the operation of a siphon; the presence of a positive atmospheric pressure is not required. (Contains 5 figures.)