A sink siphon is a device that is used to drain water from a sink or basin. It is typically located under the sink and is connected to the drain pipe. The siphon works by creating a seal around the drain hole, which prevents sewer gases from entering the room. It also prevents water from flowing back into the sink. Most sink siphons use a U-shaped tube that is filled with water, which creates a barrier that keeps sewer gases out. They are commonly found in kitchen and bathroom sinks.

Sink siphons, also known as P-traps, have several advantages:
They prevent sewer gas from entering the building by creating a water seal in the drain pipe.
They prevent unpleasant odors from coming from the drain.
They prevent small debris, such as food particles, from entering the drain and causing clogs.
They are relatively easy to install and maintain.
They are a code requirement in most building codes for the installation of plumbing fixtures.