A siphon is a tube or channel that moves liquid from a higher container to a lower one by routing it up and over a barrier between them, without any pump. Once the flow starts, it continues on its own until the liquid levels equalize or the upper container runs dry. Siphons show up everywhere, from draining a fish tank to brewing coffee to the plumbing under your sink.
How a Siphon Actually Works
There’s a common misconception that atmospheric pressure is what pushes liquid through a siphon. While air pressure plays a supporting role, gravity is the real engine. The liquid in the downward leg of the tube is heavier than the liquid in the upward leg, and as it falls under gravity, it pulls the rest of the liquid column along behind it. Think of it like a chain draped over a wheel: the heavier side pulls the lighter side up and over.
The speed of the flow depends on one thing: the vertical distance between the surface of the liquid in the upper container and the exit point of the tube. A bigger height difference means faster flow. This is why, when you siphon water out of a pool, lowering the exit end of the hose closer to the ground speeds things up considerably.
For a siphon to keep running, the liquid column inside the tube has to stay continuous. If air gets in and breaks the chain of liquid, flow stops. This is also why there’s a height limit. At sea level, when the peak of the siphon tube approaches about 10 meters above the water surface, the pressure at the top drops so low that the water essentially starts to boil at room temperature, breaking the column. A 2015 experiment published in Nature’s Scientific Reports did manage to run a siphon at 15 meters by using carefully degassed water, but under normal conditions, 10 meters is the practical ceiling.
Starting the Flow
A siphon won’t start on its own. You need to “prime” it by filling the tube with liquid first, which removes the air gap. There are several ways to do this. The most familiar is putting one end of the hose in your mouth and sucking until liquid starts flowing, then quickly dropping the hose below the source. This works fine for water but is dangerous with fuels or chemicals.
Brewing supply shops sell racking canes with a small plunger that pumps liquid into the tube to start the flow. For larger applications, like draining a pond through a long pipe, the typical approach is to close a valve at the exit end, fill the entire pipe with water through a port at the top, seal the port, and then open the valve. Once the air is out and the liquid column is unbroken, gravity takes over.
The Plumbing Connection
The curved P-trap under every sink and shower in your home is essentially a siphon turned on its head. It holds a small pool of water in its bend that acts as a seal, blocking sewer gases from rising back into your living space. The challenge is that the trap can accidentally siphon itself dry. When a large volume of water rushes down a drain, it can create negative pressure (a partial vacuum) in the pipes behind it. If that suction is strong enough, it pulls the water right out of nearby traps, destroying the seal.
This is why buildings have vent pipes, those vertical pipes that typically exit through the roof. Vents equalize air pressure on both sides of the trap seal, keeping it within a narrow range of about one inch of water column above or below atmospheric pressure. Without venting, flushing a toilet on one floor could siphon the trap dry on another floor, letting the smell of the sewer line into the room. Self-siphonage can also happen when a single fixture, like a bathtub, drains so fast that the momentum of the moving water column pulls the trap seal along with it.
Siphon Coffee Brewers
Vacuum coffee makers (also called siphon brewers) use pressure differences to move water in two directions. The device has a lower chamber sitting over a heat source and an upper chamber connected by a tube. As the lower chamber heats up, the water inside produces steam, and the rising vapor pressure pushes the liquid water up the tube into the upper chamber, where it mixes with ground coffee. Once the heat is removed, the steam in the lower chamber condenses rapidly back into liquid, creating a partial vacuum. The higher atmospheric pressure outside then forces the brewed coffee back down through a filter into the lower chamber. The whole cycle takes just a few minutes and produces a notably clean, bright cup.
Siphons in the Animal Kingdom
Many mollusks have biological siphons that serve completely different purposes but share the same name. Clams have a pair of tube-like siphons at their rear end: one draws water in (the incurrent siphon) and another expels it (the excurrent siphon). They use this flow to filter food particles and dissolved oxygen from the water. Because clams burrow into sand or mud, often only their siphons poke up into the water above, keeping the animal hidden while it feeds and breathes. Sperm also enter through the incurrent siphon during reproduction.
Cephalopods like octopuses and squid have a muscular, funnel-shaped siphon that works as a jet engine. The animal relaxes its mantle to fill the body cavity with water, then contracts powerfully to squirt that water out through the siphon. By aiming the siphon in different directions, the animal steers itself through the water. A threatened octopus can squirt dark ink into the jet stream, creating a concealing cloud to cover its escape. Even at rest, cephalopods rhythmically pulse water through the siphon to keep oxygen flowing over their gills.
Why Mouth-Siphoning Fuel Is Dangerous
Siphoning gasoline by mouth is one of the most common and risky uses of the principle. Even a tiny amount of gasoline accidentally inhaled into the lungs can cause chemical pneumonia, an inflammatory reaction that can develop over several hours. Swallowing gasoline irritates the entire digestive tract, and according to the CDC’s toxicology guidelines, as little as 350 grams (about 12 ounces) can be fatal in an adult. In children, less than half an ounce can be lethal. Symptoms of gasoline ingestion range from vomiting, drowsiness, and confusion to seizures, internal hemorrhaging, and circulatory failure. Gasoline vapors can also trigger abnormal heart rhythms. If you need to siphon fuel, a hand pump or a shaker siphon hose with a built-in check valve eliminates the need to put your mouth anywhere near the tube.
A Surprisingly Ancient Tool
Siphons have been in use for thousands of years. Ancient Egyptian tomb paintings depict what appear to be siphons used in winemaking. By the first century CE, the Greek engineer Hero of Alexandria described siphons in his text the Pneumatica, which catalogued devices powered by pressurized air and liquids. One of Hero’s designs used a series of siphons to create an installation of mechanical birds that produced burbling water sounds. The basic physics hasn’t changed since then. What made siphons useful to ancient engineers, the ability to move liquid over a barrier without mechanical parts, is exactly what makes them useful today.