Book 25, Question 74
Why don't drains make your bathroom smell like the sewer?
Under each sink is a U-shaped pipe called a trap that always holds a little water.
This water plug blocks smelly sewer gas from coming back up. Small vent pipes, often poking through the roof, let air into the drains so that they flow smoothly and the water in the trap does not get sucked out. If a drain is unused for months, the water can dry up.