I like Crease's expression for making sure that what you are conveying is at the same level that the intended audience can understand:
The challenge questions that one looks for are those that do not involve calculation but conceptualization, or a general sense of the physics involved. The student must then convey the conceptualization and how it settles the problem swiftly and succinctly at the undergraduate level. These questions therefore test what I like to call "impedance matching", or the ability to match the "load" of one's explanation to the environment in which it must be understood.
I like that expression: impedance matching. I think I'll copy it! :)