The connection between the individual sanitary equipment and the respective drain pipes consists of a curved tube called a "siphon".
Inside the siphon - which can have different shapes and be made of different materials - there is always a certain amount of water that prevents the return and the exit of unpleasant smells.
Due to their particular shape, the siphons slow down the flow of waste water and for this reason they can easily clog up.
To eliminate these unpleasant clogs, it is often sufficient to press energetically on the drain hole with the traditional plunger suction cup, after having properly plugged the "overflow" drain hole with a damp cloth.
When the sucker does not make it
When the suction cup is not sufficient, the lower part of the siphon must be unscrewed and the material obstructing it must be removed with a wire or a screwdriver.