Most of the houses in the metro Portland area were built before 1950. Which means every system in most of our housing stock is over 60 years old (if it hasn't been replaced). One of these systems is the water supply pipes.
I know you're probably like everyone else... you wake up, and stumble into the shower, and magically, water comes out of the spigot. Did you ever wonder how that works? All those pipes in your basement have water sitting in them, under pressure, just waiting for you to release it. The handle to the sink, or the shower, or any other myriad of fixtures, has a value that opens, and water runs.
There are a number of different piping systems used in construction over the years. In Portland, most of the original water supply pipes are galvanized. And galvanized plumbing itself is a bit magical. What would you say if I told you galvanized pipes are known for their "self-healing" properties? (to a certain extent) Curious, are you? Let me explain.
Let's start with the water itself: there are minerals in the water supply coming into your house. These minerals chemically react with the galvanized steel pipes. Initially, the reaction is rust, causing deterioration of the interior of the pipes. This causes the very typical loss in water pressure. If you opened up your pipes, they would look like a web of clogged arteries.
The same chemical reaction between the pipe and water can also "heal" the pipe. Because of the contact with water and the constant pressure in the pipe, when a pin size hole in the pipe appears, it will usually rust over and continue to be serviceable.
Don't get me wrong-you will still need to budget an eventual replacement of those galvanized pipes. But they're going to last a lot longer than they were supposed to because of a "magical" chemical reaction.