Additional rules to plumbing.
4. Keep your fingers out of your mouth.
5. Payday is Friday.
6. Rules 1 and 2 assume competence from the previous plumber. Not a safe assumption.
That was one of the first things I had to fix in my wife's house.
Her ex must have been dyslexic....
7. Use PVC glue in well ventilated areas (preferably outdoors)....
I guess the electrical equivalent is black=hot, white=neutral. I got an extremely painful shock one time because the moron who wired my range hood got it backwards. Years of conditioning to this idea makes you a little less cautious with the white wire, and I touched it while leaning the top of my head against the range hood. The shock I got felt like someone took a framing hammer and gave me a good whack on top of the head. I was woozy for like three minutes.