SOB or STD

A chance meeting with a fellow language teacher had us comparing notes.  Last week she said she'd taught a class about idioms, including explaining "off the hook" and how it had nothing to do with hookers, thereby leading to an explanantion of the word "hooker", which they all seriously wrote down with one elderly lady asking if there were male hookers...
So of course I told her about my lesson on the indefinite article which included why it's "an S.O.B" but "a son of a bitch"...
We're now plotting a lesson on "stuff no teacher ever told you".