When you know absolutely nothing about a skill, you are unconsciously incompetent — that is, you don’t know what you don’t know. As you learn more, you become consciously incompetent: you know what you don’t know. With training and practice you can become consciously competent, while total mastery makes you unconsciously competent, meaning that you use the skill so effortlessly that you’re not even aware you’re doing it.
Here’s the kicker: in order to teach a skill, you have to go backward, from being unconsciously competent to being consciously competent. Until you can teach it, moreover, you don’t really know what you know.