It never happens all at once. It begins as a subtle hesitation. You are standing in the shower, letting the hot water rinse away the day, and you notice the water isn't disappearing as quickly as it arrived. It begins to pool around your ankles, a tepid bath forming over your feet.
This water acts as a seal, preventing sewer gases from traveling up the pipe and into your bathroom. It is a brilliant piece of engineering, but it creates a structural weakness: a low point where water slows down. clogged drains in bath