A siphon is a tube which can be used to move water from a reservoir to another point. Siphons are capable of raising water over a barrier, which is what makes them distinctive and interesting, as well as highly useful. People have been working with siphons for thousands of years, with evidence suggesting that several ancient cultures were familiar with the basic principle of the siphon, and siphons continue to be quite useful for a wide variety of applications.
In classical use, a siphon is a flexible tube which is bent into a u-shape, although a rigid pipe can be used as well. One end of the siphon is inserted into a reservoir, and the other end is inserted into a container to catch the liquid, or left hanging, depending on what one is attempting to accomplish with the use of a siphon. Once the siphon has been started with a priming pump, it will continue to pull liquid out of the reservoir until it is removed or the reservoir is drained.
Priming involves filling the siphon with fluid so that hydrostatic pressure will come into play. One of the simplest ways to prime a siphon is to suck on it like a straw until liquid fills the tube, starting the siphoning process. It is also possible to use manual or electric pumps, which can be preferable in the case of large siphons or siphoning procedures involving toxic fluids. A siphon will run until it starts to suck air.
One of the more infamous applications of the siphon is in gas siphoning, in which people insert a tube into a gas tank and use the tube to steal gas. Many cars have anti-siphoning measures in place which are designed to prevent this activity. Siphoning is also used in a wide variety of manufacturing processes, ranging from the production of beer to the movement of various fluids along a factory line. Variations on the siphon design are also used in household plumbing.
Many people are so familiar with the siphon concept that they don't really think about how siphons work. The process actually involves a great deal of math, with siphons being capable of lifting liquids in a gravity-defying act which is rather remarkable, when one thinks about it. As long as the end of the tube is lower than the reservoir and the siphon is primed, it will run quite happily.