Communicating is one of the hardest problems to solve. We have a common representation of concepts in terms of a language, but we have no control of how exactly those language constructs map to abstract concepts in the listener’s mind (Cultura e signos).
As a general rule, make no assumptions about your audience’s background.
Nothing is obvious, I really dislike when people say that something is obvious when explaining something. We can say that, given a precondition or an existing knowledge, something is trivial, but never obvious. Saying that something is obvious does no good for any explanation, it only inflates the ego of the speaker while making the listener feel bad for not agreeing that the said thing is obvious.