What law makes a drain work?

What law makes a drain work? The law of gravity. The basic idea of how a drain works is pretty simple. Water flows downhill, it starts from the high end and runs by gravity to the lower end through a pipe. While gravity will move water down a drainpipe it will not flow smoothly without a vent. A vent allows air into the drainpipe and gases out. The slope of the pipe is also important because to much and the water moves faster than the solids leaving them behind to build up and eventually causing a problem. Standard building code requirement for drainpipes is one-quarter inch of downward slope per foot of run.