The real measure of a mentor is how many students end up passing them.

Teaching well means building people who will eventually be better than you, which requires giving away the things that made you valuable. Anyone who holds back the important parts keeps their position and produces nobody. The willingness to be overtaken is what separates a teacher from a gatekeeper.

Give away the thing you would normally keep, especially the judgment you think cannot be explained. Let the person you are helping take work that you could do better. Measure yourself by what they can do without you in two years.