The fixture above causes water to come out of the fixture below.
The fixture can be any fixture, in the animation below the fixture on the top floor is a Bathtub, and the one on the bottom is a kitchen sink, but either one of them can be any other fixture, IE a toilet shower laundry sink and so on. The black spot that appears in drain line represents a clog. the purpose of this image is to give you an idea of how it happens. In this situation all of the other toilets, sinks and tubs drain fine, to solve this problem the clog needs to be removed, this needs to be done with a snake, CHEMICALS DO NOT WORK. To see other animations showing other drain problems, please click here or hit the back button on your browser.
The image below is just an example of how drain and vent pipes may be run, the drain lines in your home may be run very different from this image. Although there are plumbing codes that give rules and regulations on how the system needs to be run, there are many ways to run the pipes with in the guidelines.