Plumbing Traps
In order to prevent sewer gasses and odors from entering the house, plumbing drains are designed with "traps."   A trap is a section of the drain pipe, usually directly under the drain, that forms an "S," and the drain pipe forms a 180-degree curve followed directly by another 180-degree curve.  The pipe can also form a "P," and the drain pipe forms a 180-degree curve followed by a 90-degree curve.  The trap or curves in the pipe are designed to allow the water to drain and remaining filled with water.  The water acts as a barrier to the sewer gasses.