Calling people stupid

[paints]

People sound stupid to me when I can't make them understand what I'm telling them that they need to know.

That doesn't mean that they are stupid. It just means that you haven't hit on the right combination of words that will make them understand what you're trying to get across to them. 
Everybody learns at their own pace, and in their own way.  Some people have to be told over and over before they get it.
Some have to have written instructions, and some learn by doing.