Plumbing siphon

A plumbing trap is a device installed under a sink or bathtub to prevent bad odors and waste water from coming up through the pipe. It usually consists of a U-shaped tank, which contains a small amount of water that creates a physical barrier between the pipe and the outside air. The trap also allows the air circulating around the pipe to accumulate particles larger than gravity can absorb, which helps keep the pipes clean and free of blockages.