What is the one idea that unlocks this?

A subject where you have learned the parts but it still hasn't clicked.

For the topic below, identify the single idea that makes everything else follow.

1. State it in one sentence.
2. Explain why it is the load-bearing idea and not just an important one.
3. Take three things that seem like separate facts or rules in this topic and show how each one is a consequence of that idea.
4. Tell me what people typically believe instead, before they get it, and why that alternative feels reasonable.
5. Give me one question I should be able to answer correctly if I have grasped it, and one I would get wrong if I have not.

Pick one idea. Do not give me a list of three candidates.

Topic:

Explaining a concept. Getting an idea across at the depth you need, with analogies, origins and first principles.