I once had a conversation with what seemed to be a reasonably savvy lady in a Doctor's surgery. She had rung to ask a question about the icons on her desktop. I narrowed down the problem to someone before her having set up Windows to respond to one click rather than the standard two.
I told her to try double-clicking the icon. It took her a while to work out what I meant. She had been told, when she started working there, that one had to "left-press" on the icon - double clicking was meaningless to her.
That's when I learnt to speak in single syllable words to users - bless 'em!