I finally figured out a way to test whether you have taught an idea or you have only taught a definition. Test it this way: You say, “Without using the new word which you have just learned, try to rephrase what you have just learned in your own language.” [… If you cannot] you learned nothing about science.

Richard Feynman on how to identify pseudoscience.

Feynman is (or, rather, was; the speech is from 1966) talking about being able to identify good versus bad science teaching for kids, but this is applicable in a lot of places.

If someone can’t describe to you a thing without using the word for the thing, they don’t know what the thing is. (See also the cloudbig data, and so on.)