Click here to purchase the entire book in PDF format. Introduction Picture it - you're getting a shower one morning and someone in the bathroom downstairs flushes the toilet... what happens? You scream in pain because you're suddenly deprived of cold water in your comfortable hot / cold mix... Not good. Why does this happen? It's simple... It's because you were forced to share cold water without being asked for permission. Essentially, we can pretend (for the purposes of this example...) that there is a steady pressure pushing the flow of water into your house. When the shower and the toilet are asking for water from the same source (the intake to your house) then the water that was normally flowing through only one source suddenly goes in two directions. The flow is split between two paths. How much water is going to flow down each path? That depends on the amount of resistance the water ``sees'' going down each path. The toilet is probably going to have a lower ``resistance'' to impede the flow of water than your shower head, and so more water flows through the toilet than the shower. If the resistance of the shower was smaller than the toilet, then you would only be mildly uncomfortable instead of jumping through the shower curtain to get away from the boiling water... In addition, the toilet would take longer to fill than it usually does when no one is showering.