Siphon

A siphon (from Ancient Greek σίφων (síphōn) ‘pipe, tube’; also spelled syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in an inverted “U” shape, which causes a liquid to flow upward, above the surface of a reservoir, with no pump, but powered by the fall of the liquid as it flows down the tube under the pull of gravity, then discharging at a level lower than the surface of the reservoir from which it came. (From wikipedia)

How does a siphon work?

There are two leading theories about how siphon works. In both theories gravity plays a role. In the first theory it is believed that the fluid that is being pulled into the lower reservoir creates a low pressure at the top of the tube, reducing the pressure below the atmospheric pressure, causing the atmospheric pressure to push the fluid into the tube. However it has been shown that siphons can work in vacuum. So scientists came up with the cohesion tension theory. According to this theory the fluid continues to flow due to cohesive forces between the atoms in the fluid.

_{Image attribution - Tomia/ CC BY-SA 3.0}

Flush toilet

One of the common every day use of the principles of siphon can be found in flush toilets.

_{Image attribution - SouthHamsian/ CC BY-SA 4.0}