Wording is so important whether it is a question, comment, request or a warning.
Here is an example (and I hope I haven't already given you this one):
"Don't use that cough as an excuse to leave class early."
Right after I said it I knew I had made a big mistake.
I could see his brain working: okay I can't use the cough so I must be able to use something else as a reason to leave class early.
In essence I just gave him permission to use something else - stomach ache, headache, whatever, as an excuse to leave class early.
And of course he used it.
I have some other things that I want to share and of course I thought I could remember and could post it later - but no not me - I have forgotten. From now on I've got to post as I think of it!
From my world to yours.......