customer: My car is running bad, change the spark plugs
me: Ok, well I noticed your check engine light is on as well. I can change your spark plugs, but that might not be the problem. You should let us put it on the scanner so we can be sure we're taking care of problem.
customer: No, just change the spark plugs.
me: Alright, but I just want you to know that changing the plugs probably won't fix it.
customer: Change the spark plugs.
**After changing the plugs**
customer: It's still doing the same thing! You didn't change anything! Show me the spark plugs you took out!!