String line levels, on a DIY project I couldn't figure out why my wife, who is very intelligent, could not understand how my string lines were correct and her "eyeing it" was wrong. It all makes much more sense to me now. And by the way, she gets it now that the project is done and the water is higher on one side than the other. Yup I said fuck-it and decided to let her see it in the finished product rather than get a divorce.

I did successfully teach my wife how to read a level. The subtleties of not stripping the fuck out of the torx head/bit while using an impact drill still escapes her though.