strange things begin to happen. for example, you may flush a toilet on the second floor of your house and the kitchen sink drain might begin to make strange sounds. the drain will sound like someone with a giant straw is sucking the water from the trap beneath the sink. in fact, that is exactly what is occurring.
this happens for a very simple reason. as the water from the toilet drains down the pipe going to the basement or sewer, it is pushing ahead of it the air that was in the pipe just before you tripped the toilet handle. this creates a vacuum just behind the water. because the 