When I worked at the Federal Court System I was often sent out to teach people. My students generally were Court Financial Administrators, Clerks of Court, and Federal Judges.
I had a slide that I almost always used at the start of a class:
Three Definitions:
Ignorance - Lack of knowledge (This is usually curable)
Stupidity - Ignorance coupled with an unwillingness to learn (Many people consider this a Capital crime)
Stupid Question - One which you do not ask as you are afraid you will show ignorance
I would then give a comment that if someone did not understand some information I was giving, there might be others with the same question; and that if asked I would re-explain the point from a different direction.
Anyway, I often had people, after the class, asking if they could have the slide or if they could make their own for use in classes which they taught. I always gave permission, and if I was not going to be using the slide again on that trip I would give it away and print another back at my office.
(And anyone who reads this, if you want to use that as your own slide, feel free to do so (just don't claim credit).)