Teach the reasoning and the answer can evolve without becoming lost.

Answers are tied to circumstances, and circumstances move. Reasoning is portable, so when the situation changes the person can derive a new answer instead of applying an old one that no longer fits. Teaching only conclusions guarantees that the knowledge expires quietly and nobody notices until it fails.

Whenever you give an answer, give a sentence of the thinking behind it. Then ask what would change if a detail were different. That question is what keeps the lesson alive.