The student should eventually see beyond the teacher’s horizon.

If a student never goes past the teacher, the field ends there. That means teaching has to include the things you are unsure of, the questions you never resolved, and enough of the underlying method that someone can eventually see what you could not. It requires a certain willingness to be surpassed by design rather than by accident.

Tell the people you teach what you never worked out. Point at the edge of what you know rather than hiding it. That edge is where their contribution starts.