[dirtbiker]

They have not the slightest idea of how plumbing, carpentry, or electrical systems work!

Plumbing is easy.

There are only 3 things you have to know about plumbing:
1. Hot is on the left,
2. Cold is on the right, and,
3. $h*t doesn't flow uphill....

[Dinsdale]

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.

[Still Thinking]

There are only 3 things you have to know about plumbing:
1. Hot is on the left,
2. Cold is on the right, and,
3. $h*t doesn't flow uphill....

Well, that's for a DIYer. As you might expect, at a professional level there's a little more to learn about the trade, and here it is:

4. Payday's Friday.

[Southbound]

No, No, No! 1. Sh*t don’t run uphill. 2. DON’T bite your fingernails. 3. Payday is Friday.

[postalx]

Continued...
4.) The boss is an a$$hole
5.) Don’t bite your fingernails. Eeeeeew!
6.) payday is Friday.

[NotQuiteCricket]

You said:
“Plumbing is easy.

There are only 3 things you have to know about plumbing:
1. Hot is on the left,
2. Cold is on the right, and,
3. $h*t doesn’t flow uphill....”

I would like to add,
And never cut an apple with your work knife.

[packrat01]

4. Don’t chew your fingernails