Explain the Difference Between Knowing Something and Understanding It

You know the speed of light is approximately 300,000 kilometres per second. You probably do not understand why that limit exists or what it means. This explains the difference between those two states, gives you a quick test to find out which one you have, and shows what understanding actually requires.

Explain the difference between knowing a fact and understanding a concept. Use the following structure:
1. The distinction: what separates knowing from understanding in plain English
2. A test: how someone can tell which one they have for a given piece of knowledge
3. An example: a commonly known fact that most people do not understand and what understanding it would actually require
4. The practical implication: what changes when you shift from knowing to understanding something

No technical terminology from learning science. Under 200 words. Write as if explaining to a curious teenager.