Drain waste traps
Every food/waste give off gas and when you flush the toilet or drain the sink or bath, the water disappears out of your house to connect somewhere outside with the main drainage. Gas will try to escape back up the pipes or through any other aperture it can get through. Every single trap starts to be useful when it has water in it. The water sits in the U bend where it forms a blockage for gas trying to get back into the room. The water will sit just below the level of the outlet bend until more water is introduced from the incoming pipe. The incoming water will replace the water in the trap and push it out of the outlet. This rule works with all traps.