A siphon is a tube that is used to move liquids between locations without any mechanical pumping. This device uses pressure to keep the liquid flowing without exerting external force.
Siphons are an effective way to pump liquid out of an area or container. For example, when irrigating a field, a siphon moves water from the water basin to the field. Similarly, siphoning is commonly used in brewing beer, when the brewer needs to transport the liquid wort between containers without introducing oxygen into the mixture.
Siphons operate using atmospheric pressure. The container where the liquid begins must be higher than the container or area where the liquid is siphoned to. This height differential ensures that the liquid flows toward the lower area; the longer length of tube in that direction ensures that gravity pulls the liquid in the correct direction. The tube must then be filled with the liquid. As the flow begins, it lowers the atmospheric pressure in the tube; the space that the liquid evacuated moves toward being a vacuum. This creates a pressure differential with the higher-pressure area of the liquid outside the tube. This pressure differential pulls the liquid into the tube to equalize the pressure. Meanwhile, gravity continues to pull more liquid out the lower end of the tube, maintaining a continuous flow through the siphon.